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[leetcode-611-Valid Triangle Number]

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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].

思路:

首先將數組排序。依次取出前兩個邊的長度。第三條邊長度小於前兩個邊的和。

查找第三條邊用二分查找。

int triangleNumber(vector<int>& a) {
      int i,j,k,n,left,right,mid,ans,sum;
      ans=0;
      sort(a.begin(),a.end());
      n
=a.size(); for (i=0;i<n;i++) for (j=i+1;j<n;j++) { sum=a[i]+a[j]; left=j;right=n; while (right-left>1) { mid=(left+right)/2; if (a[mid]>=sum) right=mid; else left=mid; } ans
+=left-j; } return ans; }

參考:

https://leetcode.com/lympanda/

[leetcode-611-Valid Triangle Number]