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)

Example: Given envelopes = [[5,4],[6,4],[6,7],[2,3]], the maximum number of envelopes you can Russian doll is 3 ([2,3] => [5,4] => [6,7]).

Solution

public class Solution {

    public class Envelope{
        public int w;
        public int h;
        public Envelope(int w, int h) {
            this.w = w;
            this.h = h;
        }
    }

    public int maxEnvelopes(int[][] envelopes) {

        // Copy for performance evaluation
            if (   envelopes           == null
            || envelopes.length    == 0
            || envelopes[0]        == null
            || envelopes[0].length == 0){
            return 0;    
        }

        Arrays.sort(envelopes, new Comparator<int[]>(){
            @Override
            public int compare(int[] e1, int[] e2){
                return Integer.compare(e1[0], e2[0]);
            }
        });

        int   n  = envelopes.length;
        int[] dp = new int[n];

        int ret = 0;
        for (int i = 0; i < n; 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], 1 + dp[j]);    
                }
            }

            ret = Math.max(ret, dp[i]);
        }
        return ret;
    }
}