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].

三角形的兩個短邊長和要大於第三個長邊長。

// Two Pointer Method
int triangleNumber(vector<int>& nums) { // time: O(n^2); space: O(1)
    sort(nums.begin(), nums.end());
    int res = 0, n = nums.size();
    for (int i = n - 1; i >= 2; --i) {
        int l = 0, r = i - 1;
        while (l < r) {
            if (nums[l] + nums[r] > nums[i]) {
                res += (r - l);
                --r;
            } else {
                ++l;
            }
        }
    }
    return res;
}