Leetcode: 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).

比较简单，O(n^2)时间复杂度。

class Solution {
public:
int threeSumClosest(vector<int> &num, int target) {
int size = num.size();
if (size < 3) {
return target;
}
sort(num.begin(), num.end());
int sum, closet_sum = num[0] + num[1] + num[2];
for (int i = 0; i < size; ++i) {
for (int j = i + 1, k = size - 1; j < k;) {
sum = num[i] + num[j] + num[k];
if (sum < target) {
++j;
}
else if (sum > target) {
--k;
}
else {
return sum;
}

if (abs(sum - target) < abs(closet_sum - target)) {
closet_sum = sum;
}
}
}

return closet_sum;
}
};