算法分析與設計——LeetCode Problem.16 3Sum Closest

題目鏈接

問題描述

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).

解題思路

與第15題3Sum類似，只要三個數的和爲target時即刻返回target；維護一個用於表示3個數的和與target的差的最小值的變量即可

代碼如下

class Solution {
public:
    int threeSumClosest(vector<int>& nums, int target) {
        int vsize = nums.size();
		sort(nums.begin(), nums.begin() + vsize);
		int closest = nums[0] + nums[1] + nums[vsize-1];
		if (closest == target) return target;
		for (int i = 0; i < vsize - 2; i++) {
			int j = i + 1, k = vsize - 1;
			while (j < k) {
				if (nums[i] + nums[j] + nums[k] == target) {
					return target;
				}
				if (nums[i] + nums[j] + nums[k] < target) {
					if (abs(closest - target) > abs(nums[i] + nums[j] + nums[k] - target)) {
						closest = nums[i] + nums[j] + nums[k];
					}
					j++;
					continue;
				}
				if (nums[i] + nums[j] + nums[k] > target) {
					if (abs(closest - target) > abs(nums[i] + nums[j] + nums[k] - target)) {
						closest = nums[i] + nums[j] + nums[k];
					}
					k--;
					continue;
				}
			}
		}
		return closest;
    }
};