[Leetcode] 16. 3Sum Closest 解题报告

题目：

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).

思路：

        思路和Leetcode 15. 3Sum十分类似。由于要求最接近target的值，所以先初始化一个result，然后不断更新即可。为了加快速度，一旦发现sum == target，就可以提前返回。时间复杂度为O(n^2)，空间复杂度为O(1)。

代码：

class Solution {
public:
int threeSumClosest(vector<int>& nums, int target)
{
if(nums.size() < 3)     return 0;

sort(nums.begin(), nums.end());
int result = nums[0] + nums[1] + nums[2];
for(int i = 0; i < nums.size(); ++i)
{
int j = i + 1;
int k = nums.size() - 1;
while(j < k)
{
int sum = nums[i] + nums[j] + nums[k];
if(sum == target)       return sum;
else if(sum < target)   ++j;
else                    --k;
if(abs(sum - target) < abs(result - target))
result = sum;
}
}
return result;
}
};