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).

与2sum，3sum类似

//http://www.cnblogs.com/remlostime/archive/2012/10/27/2742667.html

class Solution {
public:
int threeSumClosest(vector<int> &num, int target) {

sort(num.begin(), num.end());

int ret;
bool first = true;

for(int i = 0; i < num.size(); i++)
{
int j = i + 1;
int k = num.size() - 1;

while(j < k)
{
int sum = num[i] + num[j] + num[k];
//如果是第一次，先求出一个和在ret中保存下来至
if (first)
{
ret = sum;
first = false;
}
//否则，将该次循环产生的(和-target)与（ret-target进行比较）
else
{//如果该次循环的差值更小，则更新ret
if (abs(sum - target) < abs(ret - target))
ret = sum;
}

if (ret == target)
return ret;

if (sum > target)
k--;
else
j++;
}
}

return ret;
}
};