16 3Sum Closest

16. 3Sum Closest

1. Question

Given an arraySofnintegers, 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).

2. Implementation

(1) Two Pointers

class Solution {
    public int threeSumClosest(int[] nums, int target) {
        if (nums.length < 3) {
            return Integer.MAX_VALUE;
        }

        Arrays.sort(nums);
        int res = nums[0] + nums[1] + nums[2];

        for (int i = 0; i < nums.length - 2; i++) {
            int j = i + 1, k = nums.length - 1;

            while (j < k) {
                int sum = nums[i] + nums[j] + nums[k];

                if (sum == target) {
                    res = sum;
                    break;
                }
                else if (Math.abs(sum - target) < Math.abs(res - target)) {
                    res = sum;
                }

                if (sum > target) {
                    --k;
                }
                else {
                    ++j;
                }
            }
        }
        return res;
    }
}

3. Time & Space Complexity

Two Pointers: 时间复杂度O(n^2), 空间复杂度O(logn)，sorting需要logn的空间