976. Largest Perimeter Triangle ---------E ~~~~~

Q:

Given an array A of positive lengths, return the largest perimeter of a triangle with non-zero area, formed from 3 of these lengths.

If it is impossible to form any triangle of non-zero area, return 0.

 

Example 1:

Input: [2,1,2]
Output: 5

Example 2:

Input: [1,2,1]
Output: 0

Example 3:

Input: [3,2,3,4]
Output: 10

Example 4:

Input: [3,6,2,3]
Output: 8

 

Note:

1. 3 <= A.length <= 10000
2. 1 <= A[i] <= 10^6

A:

首先 all permuation .  ------- 结果 LTE

class Solution {
public:
    int largestPerimeter(vector<int>& A) {
        int res = 0;
        int n = A.size();
        sort(A.begin(), A.end());
        for(int i =0;i<n; i++){
            for(int j = i+1;j<n;j++){
                for(int k = j+1; k<n;k++){
                    if(A[i] + A[j] > A[k]){
                        res = max(res,  A[i] + A[j] + A[k] );
                    }
                }
            }
        }
        return res;
    }
};

再研读题目，发现我们应该从打的数开始比较。  而且只需要比较某个数之前2个位置就够了

class Solution {
public:
    int largestPerimeter(vector<int>& A) {
        sort(A.begin(), A.end());
        for(int i =A.size()-1;i>=2; i--){
            if(A[i-2] + A[i-1] > A[i])
                return A[i-2] + A[i-1] + A[i];
        }
        return 0;
    }
};