259. 3Sum Smaller

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题目

Given an array of n integers nums and a 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.

Example:

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

思路

跟 3Sum 类似，但是这题只用求count个数。所以当second 和 third满足条件时，[second,third]这个区间都满足条件，于是 计算count += third - second就可以了。

代码

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//C++
int threeSumSmaller(vector<int>& nums, int target) {
    if(nums.size()<3) return 0;
    int count = 0;
    sort(nums.begin(),nums.end());
    for(int first = 0;first<nums.size()-2;first++) {
        int second = first + 1;
        int third = nums.size()-1;
        while(second<third) {
            if(nums[first] + nums[second] + nums[third]<target) {
                count += third - second;
                second ++;
            }
            else {
                third --;
            }

        }
    }
    return count;
}