[leetcode]611. Valid Triangle Number有效三角数
Posted liuliu5151
tags:
篇首语:本文由小常识网(cha138.com)小编为大家整理,主要介绍了[leetcode]611. Valid Triangle Number有效三角数相关的知识,希望对你有一定的参考价值。
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].
题意:
给定数组,可由数组中的数字组成多少个valid三角形
解本题的背景知识: 【Math fact】The sum of two sides of a triangle must be greater than the third one
Solution1: Two Pointers (similar as 3 Sum problem)
1. sort array
2. lock pointer k at the trail (must be largest side)
[2, 2, 3, 4]
^k
3. set pointer left at 0, right at k-1
[2, 2, 3, 4]
^k
^left ^right
Lock pointer k and pointer right
check if nums[left] + nums[right] > nums[k]
(1)YES. Then we can say (nums[left + 1] / nums[left + 2]....) + nums[right] > nums[k]
So combination count: right - left
Then lock pointer k and pointer right -1 to get next combination count
(2) No. Then left ++
code
1 /* 2 Time: O(n^2) 3 Space: O(1) 4 */ 5 6 class Solution { 7 public int triangleNumber(int[] nums) { 8 if(nums == null || nums.length == 0) return 0; 9 Arrays.sort(nums); 10 int result = 0; 11 for (int k = nums.length - 1; k > 0 ; k--) { 12 int left = 0; 13 int right = k - 1 ; 14 while (left < right) { 15 if (nums[left] + nums[right] > nums[k]) { 16 result += (right - left); 17 right--; 18 } 19 else { 20 left++; 21 } 22 } 23 } 24 return result; 25 } 26 }
以上是关于[leetcode]611. Valid Triangle Number有效三角数的主要内容,如果未能解决你的问题,请参考以下文章
[leetcode]611. Valid Triangle Number有效三角数