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