611. Valid Triangle Number

Given an array consists of non-negative integers, your task is to count the number of triplets chosen from the array that can make triangles if we take them as side lengths of a triangle.

Example 1:

Input: [2,2,3,4]
Output: 3
Explanation:
Valid combinations are: 
2,3,4 (using the first 2)
2,3,4 (using the second 2)
2,2,3

Note:

1. The length of the given array won't exceed 1000.
2. The integers in the given array are in the range of [0, 1000].

解题思路：

别用深搜n3。确定2个数字实际上在n的时间可以解决，所以可以用n^2来解决这个问题。

class Solution {
public:
    int triangleNumber(vector<int>& nums) {
        if(nums.size()<3)return 0;
        sort(nums.begin(),nums.end());

        int max_index=nums.size()-1;
        int temp;
        int count=0;
        int now_second;
        int now_first;
        for(int i=max_index;i>=2;i--){

            now_second = i-1;
            now_first = 0;
            while(now_first<now_second){
            for(int j=now_first;j<now_second;j++){
                if(nums[j]+nums[now_second]>nums[i]){now_first = j;count+=(now_second-now_first);break;}
            }
            now_second--;
            }
            }
            return count;
    }
};