16. 3Sum Closest

Description

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).

Solution

本题和15. 3Sum不同，不是求很多组结果，而是一个最和target接近的三数之和sum而已，结果唯一，同样的步骤，做好中间结果的保存

int threeSumClosest(vector<int>& nums, int target) {
    int closestSum = nums[0] + nums[1] + nums[2];
    sort(nums.begin(), nums.end());
    for (int i = 0; i < nums.size() - 2; ++i) {
        if (target < 0 && nums[i] > 0) {//提前剪枝
            break;
        }
        int begin = i + 1, end = nums.size() - 1;
        while (begin < end) {
            int sum = nums[i] + nums[begin] + nums[end];
            if (abs(closestSum - target) > abs(sum - target)) { //如果出现更接近的sum
                closestSum = sum;
            }
            if (sum > target) {
                end--;
            } else if (sum < target) {
                begin++;
            } else {
                return target;
            }
        }
    }
    return closestSum;
}