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)

Have you met this question in a real interview?  
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]).

//good explain:https://www.jianshu.com/p/d74fa8ede028
public static int maxEnvelopes(int[][] envelopes) {
        if (envelopes == null || envelopes.length == 0)
            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]; // ascending order
                } else {
                    return b[1] - a[1]; // descending order
                }
            }
        });
        ArrayList<Integer> list = new ArrayList<Integer>();
        //遍历每个信封，相当于决定每个信封的位置
        for (int i = 0; i < envelopes.length; i++) {

            if (list.size() == 0 || list.get(list.size() - 1) < envelopes[i][1]) {
                list.add(envelopes[i][1]);
                continue;
            }
            //二分替换，使得加进来的每个信封的宽在List集合中是升序排序
            int l = 0;
            int r = list.size() - 1;
            while (l < r) {
                int m = (l + r) / 2;
                if (list.get(m) < envelopes[i][1]) {
                    l = m + 1;
                } else {
                    r = m;
                }
            }
            list.set(r, envelopes[i][1]);
        }

        return list.size();
    }