16. 3Sum Closest

Medium

Given an integer array nums of length n 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).

Example 2:

Input: nums = [0,0,0], target = 1
Output:
 0
Explanation:
 The sum that is closest to the target is 0. (0 + 0 + 0 = 0).

Constraints:

• 3 <= nums.length <= 500

• -1000 <= nums[i] <= 1000

• -104 <= target <= 104

解題

Runtime: 20 ms, faster than 95.35%

Memory Usage: 3.2 MB, less than 85.45%

func threeSumClosest(nums []int, target int) int {
    res := nums[0] + nums[1] + nums[2] 

    sort.Ints(nums)

    for i, num := range nums {
        left := i+1 //從 i+1 開始，因為 i 前面的已經跑過了
        right := len(nums) - 1 

        for left < right {
            sum := num + nums[left] + nums[right]
            if abs(sum - target) < abs(res - target) {
                res = sum
            }

            if sum == target {
                return sum
            } else if sum < target { // 比目標小，將 left 右移，下一次的sum會變大
                left++
            } else {
                right-- //下一次的sum會變小
            }
        }
    }

    return res
}

func abs(a int) int {
    if a < 0 { return -a }
    return a
}