Leetcode 354. Russian Doll Envelopes

You have a number of envelopes with widths and heights given as a pair of integers (w, h). One envelope can fit into another if and only if both the width and height of one envelope is greater than the width and height of the other envelope.

What is the maximum number of envelopes can you Russian doll? (put one inside other)

Note:
Rotation is not allowed.

Example:

Input: [[5,4],[6,4],[6,7],[2,3]]
Output: 3 
Explanation: The maximum number of envelopes you can Russian doll is 3 ([2,3] => [5,4] => [6,7]).

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Analysis:
The problem is simply a Longest Increasing Subsequence problem. 

Code (Java):

class Solution {
    public int maxEnvelopes(int[][] envelopes) {
        if (envelopes == null || envelopes.length == 0) {
            return 0;
        }
        
        Arrays.sort(envelopes, new EnvelopesComparator());
        int[] dp = new int[envelopes.length];
        int maxLen = 1;
        
        for (int i = 0; i < envelopes.length; i++) {
            dp[i] = 1;
            for (int j = 0; j < i; j++) {
                if (envelopes[i][0] > envelopes[j][0] &&
                    envelopes[i][1] > envelopes[j][1]) {
                    dp[i] = Math.max(dp[i], dp[j] + 1);
                    maxLen = Math.max(maxLen, dp[i]);
                }
            }
        }
        
        return maxLen;
    }
}

public class EnvelopesComparator implements Comparator<int[]> {
    @Override
    public int compare(int[] a, int[] b) {
        if (a[0] - b[0] != 0) {
            return a[0] - b[0];
        }
        
        return a[1] - b[1];
    }
}