259. 3Sum Smaller

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 < nthat 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;
}
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