LeetCode [259] 3Sum Smaller

 259. 3Sum Smaller

Medium

Given an array of n integers nums and an integer target, find the number of index triplets i, j, k with 0 <= i < j < k < n that satisfy the condition nums[i] + nums[j] + nums[k] < target.

Follow up: Could you solve it in O(n2) runtime?

 

Example 1:

Input: nums = [-2,0,1,3], target = 2
Output: 2
Explanation: Because there are two triplets which sums are less than 2:
[-2,0,1]
[-2,0,3]

Example 2:

Input: nums = [], target = 0
Output: 0

Example 3:

Input: nums = [0], target = 0
Output: 0

 

Constraints:

• n == nums.length
• 0 <= n <= 300
• -100 <= nums[i] <= 100
• -100 <= target <= 100

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class Solution { public: int threeSumSmaller(vector<int>& nums, int target) { sort(nums.begin(), nums.end()); int sz = nums.size(), ret = 0; for(int i=0; i<=sz-3; ++i){ int l = i+1, r = sz-1; while(l<r){ int sum = nums[i]+nums[l]+nums[r]; if(sum>=target){ r--; }else{ ret += (r-l); l++; } } } return ret; } };