俄罗斯套娃信封问题

class Solution {
 public:
  int maxEnvelopes(vector<vector<int>>& envelopes) {
    int n = envelopes.size(), maxLen = 1;
    vector<int> dp(n + 1);
    sort(envelopes.begin(), envelopes.end());
    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] = max(dp[i], dp[j] + 1);
          if (dp[i] > maxLen) {
            maxLen = dp[i];
          }
        }
      }
    }
    return maxLen;
  }
};