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

Given array S = [3,4,6,7], return 3. They are:

[3,4,6]
[3,6,7]
[4,6,7]

Given array S = [4,4,4,4], return 4. They are:

[4(1),4(2),4(3)]
[4(1),4(2),4(4)]
[4(1),4(3),4(4)]
[4(2),4(3),4(4)]


先sort 然后对于每一个元素i 利用左右双index扫面0到i-1 找出符合条件的元素对

public class Solution {
    /**
     * @param S: A list of integers
     * @return: An integer
     */
    public int triangleCount(int S[]) {
        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]) {
                    // The edge from S[left] to S[right - 1] will also form a triangle
                    count += right - left;
                    right--;
                } else {
                    left++;
                }
            }
        }
        return count;
    }
}