646. Maximum Length of Pair Chain（medium）

You are given n pairs of numbers. In every pair, the first number is always smaller than the second number.

Now, we define a pair (c, d) can follow another pair (a, b) if and only if b < c. Chain of pairs can be formed in this fashion.

Given a set of pairs, find the length longest chain which can be formed. You needn't use up all the given pairs. You can select pairs in any order.

Example 1:

Input: [[1,2], [2,3], [3,4]]
Output: 2
Explanation: The longest chain is [1,2] -> [3,4]

Note:

1. The number of given pairs will be in the range [1, 1000].

dp的思想

class Solution {
public:
    int findLongestChain(vector<vector<int>>& pairs) {
        int len = pairs.size();
        if (len == 0){
            return 0;
        }
        int res = 0;
        sort(pairs.begin(), pairs.end(), cmp);
        vector<int> dp(len, 1);
        for (int i = 0; i < len; i++){
            for (int j = 0; j < i; j++){
                if (pairs[i][0] > pairs[j][1]){
                    dp[i] = max(dp[i], dp[j] + 1); 
                }
            }
        }
        return dp[len - 1];
    }
private:
    static bool cmp(vector<int>& a, vector<int>&b){
        return a[0] < b[0];
    }
};