# LeetCode 91 Decode Ways Java Solution

2023 Jun 26

### The Problem

A message containing letters from `A-Z` can be encoded into numbers using the following mapping:

``````  'A' -> "1"
'B' -> "2"
...
'Z' -> "26"
``````

To decode an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways). For example, `"11106"` can be mapped into:

• `"AAJF"` with the grouping `(1 1 10 6)`
• `"KJF"` with the grouping `(11 10 6)`

Note that the grouping `(1 11 06)` is invalid because `"06"` cannot be mapped into `'F'` since `"6"` is different from `"06"`.

Given a string `s` containing only digits, return the number of ways to decode it.

Check LeetCode link for more details and example input & output.

### A Recursive Solution

Below is a straightforward recursive solution.

``````  public int numDecodings(String s) {
return numberOfDecodingStartAt(s, 0);
}

int numberOfDecodingStartAt(String s, int index) {
if (index == s.length()) {
return 1;
}

int result = 0;
if (isChar(s, index, index + 1)) {
result += numberOfDecodingStartAt(s, index + 1);
}
if (index + 2 <= s.length() && isChar(s, index, index + 2)) {
result += numberOfDecodingStartAt(s, index + 2);
}
return result;
}

boolean isChar(String s, int start, int end) {
// "06" cannot be mapped into 'F' since "6" is different from "06".
if (s.charAt(start) == '0') {
return false;
}

int code = Integer.parseInt(s.substring(start, end));
return code > 0 && code < 27; // 1 - 26
}
``````

### Dynamic Programming Solutions

#### A Top Down Approach

Improve the performance by caching the results of sub problems.

# LeetCode 213 House Robber II Java Solution

2023 Jun 26

### The Problem

You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed. All houses at this place are arranged in a circle. That means the first house is the neighbor of the last one. Meanwhile, adjacent houses have a security system connected, and it will automatically contact the police if two adjacent houses were broken into on the same night.

Given an integer array `nums` representing the amount of money of each house, return the maximum amount of money you can rob tonight without alerting the police.

Check LeetCode link for more details and example input & output.

### A Dynamic Programming Solution

It’s not allowed to rob both house `0` and house `nums.length - 1`, because they are adjacent hourses. So the problem becomes finding the greater amount between robbing houses except the first and robbing house except the last.

The two sub problems, robbing houses except the first and robbing house except the last, can be solved using a bottom up approach described in this post.

A Java solution is listed below.

``````  public int rob(int[] nums) {
if (nums.length == 1) { // an edge case
return nums[0];
}
return Math.max(rob1(nums, 0, nums.length - 2), rob1(nums, 1, nums.length - 1));
}

/*
This method is to solve the 'house robber I' problem.
Houses are located in `nums`, from `start` to `end`.
*/
int rob1(int[] nums, int start, int end) {
int r1 = nums[end];
int r2 = 0;
for (int i = end - 1; i >= start; i--) {
int tmp = r1;
r1 = Math.max(nums[i] + r2, r1);
r2 = tmp;
}
return r1;
}
``````

The time complexity is `O(n)`.

### A Recursive Approach

Below is a straightforward recursive solution.

# LeetCode 300 Longest Increasing Subsequence Java Solution

2023 Jun 25

### The Problem

Given an integer array `nums`, return the length of the longest strictly increasing subsequence.

Check LeetCode link for more details and example input & output.

### Dynamic Programming Solutions

#### A Recursive Approach

Comparing with the original problem, it’s easier to solve the longest increasing subsequence which starts at the index `i` recursively. Let’s call it `LIS[i]`, longest increasing subsequence of `nums` starting at the index `i`.

The original problem becomes finding the maximum value among `LIS[0]`, `LIS[1]`, …, `LIS[nums.length - 1]`.

``````  public int lengthOfLIS(int[] nums) {
int max = 1;
for (int i = 0; i < nums.length; i++) {
max = Math.max(max, lengthOfLISStartAtIndex(nums, i));
}
return max;
}

int lengthOfLISStartAtIndex(int[] nums, int index) {
if (index == nums.length - 1) { // the base case
return 1;
}

int result = 1;
for (int i = index + 1; i < nums.length; i++) {
// this test guarantees getting an increasing sub-sequence
if (nums[i] > nums[index]) {
result = Math.max(result, 1 + lengthOfLISStartAtIndex(nums, i));
}
}
return result;
}
``````

#### A Top Down Approach

Improve the recursive solution with caching.

``````  public int lengthOfLIS(int[] nums) {
Map<Integer, Integer> cache = new HashMap<>();
int result = 1;
for (int i = 0; i < nums.length; i++) {
result = Math.max(result, lengthOfLIS(nums, i, cache));
}
return result;
}

// length Of LIS starting at `index`
int lengthOfLIS(int[] nums, int index, Map<Integer, Integer> cache) {
if (index == nums.length - 1) {
return 1;
}

if (cache.get(index) != null) {
return cache.get(index);
}

int result = 1;
for (int i = index + 1; i < nums.length; i++) {
if (nums[i] > nums[index]) {
result = Math.max(result, 1 + lengthOfLIS(nums, i, cache));
}
}
cache.put(index, result);
return result;
}
``````

#### A Bottom Up Approach

It’s easy to calculate `LIS[i]` using a bottom up approach too.

`LIS[i]` is the maximum of below values.

• 1 (since at least `nums[i]` is a candidate of LIS starting at `i`)
• `1 + LIS[i+1]`, if `nums[i]` < `nums[i+1]`
• `1 + LIS[i+2]`, if `nums[i]` < `nums[i+2]`

Base case is `LIS[nums.length - 1]` is `1`.

Below is a Java solution.

# LeetCode 139 Word Break Java Solution

2023 Jun 23

### The Problem

Given a string `s` and a dictionary of strings `wordDict`, return `true` if `s` can be segmented into a space-separated sequence of one or more dictionary words.

Note that the same word in the dictionary may be reused multiple times in the segmentation.

Check LeetCode link for more details and example input & output.

### Dynamic Programming Solutions

#### A Top Down Approach

``````  public boolean wordBreak(String s, List<String> wordDict) {
return wordBreak(s, 0, wordDict, new Boolean[s.length()]);
}
boolean wordBreak(String s, int index, List<String> wordDict, Boolean[] dp) {
if (index == s.length()) {
return true;
} else if (index > s.length()) {
return false;
}
if (dp[index] != null) {
return dp[index];
}
for (String word: wordDict) {
if (index + word.length() > s.length()
|| !s.substring(index, index + word.length()).equals(word)) {
continue;
}
if (wordBreak(s, index + word.length(), wordDict, dp)) {
dp[index] = true;
return true;
}
}
dp[index] = false;
return false;
}
``````

#### A Bottom Up Approach

`dp[i]` is a sub problem, whose input is a sub string of `s`, starting at the index `i`.

`dp[i]` is true, if there is at least one word `w` in `wordDict`, that makes `dp[i + len(w)]` true.

The base case is that `dp[len(s)]` is true. And, `dp[0]` is the original problem.

Below is a Java solution.

# LeetCode 62 Unique Paths Java Solution

2023 Jun 21

### The Problem

There is a robot on an `m x n` grid. The robot is initially located at the top-left corner (i.e., `grid[0][0]`). The robot tries to move to the bottom-right corner (i.e., `grid[m - 1][n - 1]`). The robot can only move either down or right at any point in time.

Given the two integers `m` and `n`, return the number of possible unique paths that the robot can take to reach the bottom-right corner.

Check LeetCode link for more details and example input & output.

### Dynamic Programming Solutions

Solve it use the bottom up approach.

``````    dp(m, n) = dp(m, n - 1) + dp(m - 1, n)
``````

`dp(m, n)` is the problem with a `m x n` grid as the input.

The answer of `dp(m, n)` is the sum of `dp(m, n - 1)` and `dp(m - 1, n)`, where `dp(m, n - 1)` means the robot chooses to move right and `dp(m - 1, n)` means the robot chooses to move down.

Base cases:

• `dp(i, 1)` is `1`. That is if the grid is a one-column grid, the robot only has one choice, which is going down.
• `dp(1, i)` is `1` too. On a one-row grid, the robot can only go right.

The sub problem `dp(i, j)` has two dimensions, so it’s a 2D dynamic programming problem. We can use a matrix, i.e. a 2d array.

``````  public int uniquePaths(int m, int n) {
// dp[i][j] is the answer for dp(i+1, j+1)
int[][] dp = new int[m][n];
// base case: dp(1, i) = 1
for (int i = 0; i < n; i++) {
dp[0][i] = 1;
}
// base case: dp(i, 1) = 1
for (int i = 0; i < m; i++) {
dp[i][0] = 1;
}
for (int i = 1; i < m; i++) {
for (int j = 1; j < n; j++) {
dp[i][j] = dp[i - 1][j] + dp[i][j - 1];
}
}
return dp[m - 1][n - 1];
}
``````

Actually, when calculate `dp[i][j]`, we only need the “previous” row in the matrix, i.e. `dp[i - 1][j]`. So we can save some space, by using only two rows. See Java code below.

# LeetCode 55 Jump Game Java Solution

2023 Jun 21

### The Problem

You are given an integer array `nums`. You are initially positioned at the array’s first index, and each element in the array represents your maximum jump length at that position.

Return `true` if you can reach the last index, or `false` otherwise.

Check LeetCode link for more details and example input & output.

### Dynamic Programming Solutions

#### A Top Down Approach

A Java solution.

``````  public boolean canJump(int[] nums) {
return canJump(nums, 0, new Boolean[nums.length]);
}

/*
starting at position `index`, return `true` if it can reach the end.
*/
boolean canJump(int[] nums, int index, Boolean[] dp) {
// if we reach the end, i.e. the last index
if (index == nums.length - 1) {
dp[index] = true;
return true;
}
// check the cache
if (dp[index] != null) {
return dp[index];
}

// try all possible jumps from the position of the `index`
for (int i = 1; i <= nums[index]; i++) {
if (canJump(nums, index + i, dp)) {
dp[index] = true;
return true;
}
}
// tried all possible jumps and unable to reach the end,
// therefore, return false when getting here
dp[index] = false;
return false;
}
``````

# LeetCode 322 Coin Change Java Solution

2023 Jun 20

### The Problem

You are given an integer array `coins` representing coins of different denominations and an integer `amount` representing a total amount of money.

Return the fewest number of coins that you need to make up that amount. If that amount of money cannot be made up by any combination of the coins, return `-1`.

You may assume that you have an infinite number of each kind of coin.

Check LeetCode link for more details and example input & output.

### Dynamic Programming Solutions

#### A Top Down Approach

A Java solution.

``````  public int coinChange(int[] coins, int amount) {
return coinChange(coins, amount, new HashMap<>());
}

int coinChange(int[] coins, int amount, Map<Integer, Integer> dp) {
if (amount < 0) {
return -1;
}
if (amount == 0) { // a base case & edge case
return 0;
}
if (dp.get(amount) != null) {
return dp.get(amount);
}

int min = amount + 1; // ①
for (int i = 0; i < coins.length; i++) {
int changes = 1 + coinChange(coins, amount - coins[i], dp);
// `changes == 0` meaning the above `coinChange` call return `-1`,
// no need to update `min` in this case
if (changes != 0 && changes < min) {
min = changes;
}
}
// if `min` stays at its initial value, it means no answer for input `amount`
if (min == amount + 1) {
min = -1;
}

dp.put(amount, min);

return min;
}
``````

For initial value of `min`, (line ①), `amount + 1` is big enough. Because even if coin `1` exists, the maximum value of `changes` is `amount`.

# LeetCode 198 House Robber Java Solution

2023 Jun 20

### The Problem

You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed, the only constraint stopping you from robbing each of them is that adjacent houses have security systems connected and it will automatically contact the police if two adjacent houses were broken into on the same night.

Given an integer array `nums` representing the amount of money of each house, return the maximum amount of money you can rob tonight without alerting the police.

Check LeetCode link for more details and example input & output.

### A Dynamic Programming Solution

Use the bottom up approach.

`dp[i] = max(nums[i] + dp[i-2], dp[i-1])`

`dp[i]` is the answer of the sub problem “i”, whose input array is a sub-array of the orginal input `nums`. The sub-array starts at index `0` and ends at index `i`.

Base cases:

• `dp[0]` is `nums[0]`.
• `dp[-1]` is 0. (It’s used to calculate `dp[1]`.)

#### Java Code

``````  public int rob(int[] nums) {
int dp1 = nums[0]; // dp[i-1]
int dp2 = 0; // dp[i-2]
for (int i = 1; i < nums.length; i++) {
int tmp = dp1;
dp1 = Math.max(nums[i] + dp2, dp1);
dp2 = tmp;
}
return dp1;
}
``````

The time complexity is `O(n)`.