Triangle Count

Given an array of integers, how many three numbers can be found in the array, so that we can build an triangle whose three edges length is the three numbers that we find?

Example

Example 1:

Input: [3, 4, 6, 7]
Output: 3
Explanation:
They are (3, 4, 6), 
         (3, 6, 7),
         (4, 6, 7)

Example 2:

Input: [4, 4, 4, 4]
Output: 4
Explanation:
Any three numbers can form a triangle. 
So the answer is C(3, 4) = 4

思路：构成三角形的条件是：任意两边之和大于第三边； 

那么我固定最长边A[i] ，剩下的任务就是从0 ~ i -1 找到两个数，和要大于A[i];

public class Solution {
    /**
     * @param S: A list of integers
     * @return: An integer
     */
    public int triangleCount(int[] S) {
        if(S == null || S.length == 0) {
            return 0;
        }
        Arrays.sort(S);
        int count = 0;
        for(int i = 0; i < S.length; i++) {
            int left = 0;
            int right = i - 1;
            while(left < right) {
                if(S[left] + S[right] > S[i]) {
                    count += right - left;
                    right--;
                } else {
                    left++;
                }
            }
        }
        return count;
    }
}