[LeetCode] 16. 3Sum Closest

Given an array nums of n integers and an integer target, find three integers in nums such that the sum is closest to target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

Example 1:

Input: nums = [-1,2,1,-4], target = 1
Output: 2
Explanation: The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

Constraints:

• 3 <= nums.length <= 10^3
• -10^3 <= nums[i] <= 10^3
• -10^4 <= target <= 10^4

最近三数之和。题意是给一个数组和一个target数字，返回一个三数之和使得nums[A] + nums[B] + nums[C] = target或尽可能接近target。

思路跟15题类似，也是需要先排序。排序之后，先固定第一个数字，然后后两个数字之间做two pointer逼近。逼近的方式是如果

res = nums[A] + nums[B] + nums[C] = target，则直接返回这个res；

如果res大于target，C--

如果res小于target，B++

时间O(n^2)

空间O(1)

JavaScript实现

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number}
 */
var threeSumClosest = function(nums, target) {
    nums = nums.sort((a, b) => a - b);
    const len = nums.length;
    let res = nums[0] + nums[1] + nums[len - 1];
    let low, high, sum;
    for (let i = 0; i < nums.length - 2; i++) {
        low = i + 1;
        high = nums.length - 1;
        while (low < high) {
            sum = nums[i] + nums[low] + nums[high];
            if (Math.abs(target - sum) < Math.abs(target - res)) {
                res = sum;
            }
            if (sum > target) {
                high--;
            } else {
                low++;
            }
        }
    }
    return res;
};

Java实现

class Solution {
    public int threeSumClosest(int[] nums, int target) {
        int res = nums[0] + nums[1] + nums[nums.length - 1];
        Arrays.sort(nums);

        for (int i = 0; i < nums.length - 2; i++) {
            int start = i + 1;
            int end = nums.length - 1;
            while (start < end) {
                int sum = nums[i] + nums[start] + nums[end];
                if (sum > target) {
                    end--;
                } else {
                    start++;
                }
                if (Math.abs(target - sum) < Math.abs(target - res)) {
                    res = sum;
                }
            }
        }
        return res;
    }
}

LeetCode 题目总结