> For the complete documentation index, see [llms.txt](https://nataliekung.gitbook.io/ladder_code/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nataliekung.gitbook.io/ladder_code/expedia-oa/valid-triangle-number.md).

# Valid Triangle Number

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

**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.
```

**Example 2:**

```
Input: [3,3,3]
Output: 1
Explanation: 
Valid combination is 3,3,3.
```

#### Notice

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]`.

分析

3sum，当前i是最大，left,right在0-i-1之间，只需要判断a+b>c

```
class Solution:
    """
    @param nums: the given array
    @return:  the number of triplets chosen from the array that can make triangles
    """
    def triangleNumber(self, nums):
        # Write your code here
        n = len(nums)
        res = 0
        nums.sort()
        for i in range(n-1,1,-1):
            l,r =0,i-1
            while l < r:
                if nums[l] + nums[r] > nums[i]:
                    res += r-l
                    r-=1
                else:
                    l+=1
        return res
                    
            
                
                
            

```


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