[leetcode]646. Maximum Length of Pair Chain

题目链接：https://leetcode.com/problems/maximum-length-of-pair-chain/#/description

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].

思路：

1. sort the pairs by their end in increasing order;
2. at any point, choose the pair that start after the tail end, then use it as the new tail;

class Solution {
public:
    int findLongestChain(vector>& pairs) {
        int res=0;
        if(pairs.empty())
            return res;
        sort(pairs.begin(),pairs.end(),cmp);
        vector boun=pairs[0];
        for(int i=0;iboun[1])
            {
                boun=pairs[i];
                res++;
            }
        }
        return res;
    }
private:
    static bool cmp(vector& a,vector& b)
    {
       return (a[1]