611. Valid Triangle Number

Posted 2020-10-11 andywu

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]. 

找出满足组成三角形的三条边的个数

 思路：组成三角形条件是两条较短的边的和大于第三条边

 1     public int triangleNumber(int[] nums) {
 2         Arrays.sort(nums);
 3         int count = 0;
 4         for (int i = nums.length - 1; i >= 2; i--) {
 5             int left = 0, right = i - 1;
 6             while (left < right) {
 7                 if (nums[left] + nums[right] > nums[i]) { //最短的两条边之和大于最长的边nums[i]
 8                     count += right - left; //[left.right-1) 区间里面的数+right都满足条件
 9                     right--;
10                 } else {
11                     left++;
12                 }
13             }
14         }
15         return count;
16     }