[LeetCode] 354. Russian Doll Envelopes

Problem

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]).

Solution

class Solution {
    public int maxEnvelopes(int[][] envelopes) {
        if (envelopes == null || envelopes.length == 0 ||
           envelopes[0] == null || envelopes[0].length != 2) return 0;
        
        Arrays.sort(envelopes, new Comparator<int[]>(){
            public int compare(int[] a, int[] b) {
                if (a[0] != b[0]) return a[0]-b[0];
                else return b[1]-a[1];
            }
        });
        
        int[] dp = new int[envelopes.length];
        int len = 0;
        
        //find the position to be inserted
        //if the height is smaller than current high, just update smaller index
        //if the height is larger than current high:
        //  it will be the length 'len' (and happen to be the 
        //  highest index+1 ==> next index), so we insert it into 
        //  the next index in dp[] and update effective length 'len'
        for (int[] envelope: envelopes) {
            int index = Arrays.binarySearch(dp, 0, len, envelope[1]);
            if (index < 0) index = -index-1;
            dp[index] = envelope[1];
            if (index == len) len++;
        }
        
        return len;
    }
}