3Sum Closest -- leetcode

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本文介绍了一个算法问题：给定一个整数数组和一个目标值，找到数组中三个整数使得它们的和最接近目标值，并返回这三个数的和。通过排序和双指针技巧实现了高效的解决方案。

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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).

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) {
        if (num.size() < 3) return -1;

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

        int closest = num[0] + num[1] + num[2];
        for (int i=0; i<num.size(); i++) {
                int p = i+1, q = num.size()-1;
                while (p < q) {
                        const int diff = num[i] + num[p] + num[q] - target;
                        if (abs(diff) < abs(closest-target))
                                closest = diff + target;

                        if (!diff)
                                return closest;
                        else if (diff < 0)
                                ++p;
                        else
                                --q;
                }
        }
        return closest;
    }
};