[LeetCode] 611. Valid Triangle Number_Medium tag: Two pointers

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].

brute force: T: O(n ^3)   S: O(1)

这个题目最优解利用[LeetCode] 167. Two Sum II - Input array is sorted_Easy tag: Hash/Two pointers的做法，先将nums sort，然后将nums[2:] 中的每个数num，来判断前面有多少对数可以和num来组成triangle. Note: 这里利用了，三个数 a <= b <= c ， if a + b > c， 那么一定可以成为triangle. 

成为triangle的条件： a + b > c   & a + c > b & b + c > a.

优化后： O(n ^ 2), S: O(1)

Code

class Solution:
    def validTriNum(self, nums):
        ans = 0
        for i in range(2, len(nums)):
            left, right = 0, i - 1
            while left < right:
                if nums[left] + nums[right] > target:
                    ans += right - left
                    right -= 1
                else:
                    left += 1
        return ans