Leetcode 16 - 3Sum Closest

Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

For example, given array S = {-1 2 1 -4}, and target = 1.

The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

1 - 先对数组进行排序。

2 - 使用夹逼法：初始时，固定第一个数a的位置，将第二个数b的位置设置在a之后，将第三个数c的位置设置在数组末尾。然后根据a+b+c和target的比较结果，决定移动b的位置还是c的位置。

class Solution {
public:
int threeSumClosest(vector<int>& nums, int target) {
int result = 0;
int gap_min = INT_MAX;
sort(nums.begin(),nums.end());
//初始化a，b，c三个数的位置
for(auto a=nums.begin();a!=prev(nums.end(),2);a++){
auto b = next(a);
auto c = prev(nums.end());
while(b<c){
const int sum = *a+*b+*c;
const int gap = abs(sum-target);
if(gap<gap_min){
result = sum;
gap_min = gap;
}
//左右夹逼
if(sum>target){
c--;
}else{
b++;
}
}
}
return result;
}
};