Leetcode——611. Valid Triangle Number

题目原址

https://leetcode.com/problems/valid-triangle-number/description/

题目描述

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:

• The length of the given array won’t exceed 1000.
• The integers in the given array are in the range of [0, 1000].

解题思路

题目要求：找出满足组成三角形的三条边的个数

这个题可以归结为三个指针的问题，通过一层循环，又将三个指针问题变为两个指针问题，最后求解。

• 首先要将数组进行排序
• 排序后从数组的最右边的元素开始进行循环right，一个变量指针指向数组的第一个元素left，一个变量指针指向数组当前循环元素的前一个元素medium
• 如果满足nums[left] + nums[medium] > nums[right]，则说明在left到right中的所有元素都满足可以组成三角形的条件，所以将能够组成三角形的个数增加medium - left个
• 如果不满足上述式子，则将left向前移动

AC代码

class Solution {
    public int triangleNumber(int[] nums) {
        if(nums == null || nums.length < 3)
            return 0;
        int ret = 0;
        Arrays.sort(nums);
        for(int right = nums.length - 1; right >= 2; right--) {
            int left = 0, medium = right - 1;
            while(left < medium) {
                if(nums[left] + nums[medium] > nums[right]) {
                    ret += medium - left;
                    medium --;
                }else {
                    left ++;
                }               
            }
        }
        return ret;   
    }
}