[LintCode] 59 3Sum Closest 解题报告

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.


Notice
You may assume that each input would have exactly one solution.



Example
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).



Challenge
O(n^2) time, O(1) extra space



思路
先排序，然后遍历数组，对于每一个元素，用对撞指针来找有没有和target更近的sum。
注意closestSum初始化不能是Integer.MAX_VALUE。因为如果碰到-1，算abs的时候就越界了。



Code

public class Solution {
    /**
     * @param numbers: Give an array numbers of n integer
     * @param target : An integer
     * @return : return the sum of the three integers, the sum closest target.
     */
    public int threeSumClosest(int[] numbers, int target) {
        // write your code here
        
        if (numbers == null || numbers.length < 3) {
            return -1;
        }
        
        Arrays.sort(numbers);
        
        int closestSum = numbers[0] + numbers[1] + numbers[2];
        for (int i = 0; i < numbers.length; i++) {
            int left = i + 1;
            int right = numbers.length - 1;
            while (left < right) {
                int sum = numbers[i] + numbers[left] + numbers[right];
                if (Math.abs(sum - target) < Math.abs(closestSum - target)) {
                    closestSum = sum;
                }
                if (sum < target) {
                    left++;
                }
                else {
                    right--;
                }
            }
        }
        
        return closestSum;
    }
}