3Sum Closest 找3个数使得和最接近的target @LeetCode

思路：

与上题差不多，只是不能再用系统自带的二分法，要自己写一个。

package Level3;

import java.util.Arrays;

/**
 * 3Sum Closest 
 * 
 * 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).
 */
public class S16 {

	public static void main(String[] args) {
		int[] num = {-1,2,1,-4};
		System.out.println(threeSumClosest(num, 1));
	}
	
	public static int threeSumClosest(int[] num, int target) {
		int sum = 0;
		int diff = Integer.MAX_VALUE;
		Arrays.sort(num);

		for (int i = 0; i < num.length - 1; i++) {		// 前指针向后
			for (int j = num.length - 1; j > i+1; j--) {	// 后指针向前
				int remain = target - (num[i] + num[j]);
				int third = bsearch(num, i + 1, j, remain);
				int tmp = num[i] + num[third] + num[j];
				if(Math.abs(tmp-target) < diff){		// 保存最小不同的数
					diff = Math.abs(tmp-target);
					sum = tmp;
				}
			}
		}
		
		return sum;
    }
	
	// 修改版二分法
	public static int bsearch(int[] num, int low, int high, int key){
		int mid = low + (high-low)/2;
		while(low < high){		// low不能与high相等！
			mid = low + (high-low)/2;
			if(num[mid] == key){
				return mid;
			}else if(key < num[mid]){
				high = mid-1;
			}else{
				low = mid+1;
			}
		}
		return mid;
	}

}

Rewrite:

public int threeSumClosest(int[] num, int target) {
    if (num.length<3) { // if less than three items then return 0
        return Integer.MAX_VALUE;
    }
    Arrays.sort(num);
    int res = num[0]+num[1]+num[2];
    for (int i=0; i<num.length-2; ++i) {
        if (i>0 && num[i]==num[i-1])  continue;
        int start = i+1, end = num.length-1;
        while (start<end) {
            int sum = num[i] + num[start] + num[end];
            if (Math.abs(sum-target) < Math.abs(res-target)) {
                res = sum;
            }
            if (sum == target) {
                return res;
            } else if (sum < target) {
                start++;
            } else {
                end--;
            }
        }//end-while
    }
    return res;
}

public class Solution {
    public int threeSumClosest(int[] num, int target) {
        Arrays.sort(num);
        int diff = Integer.MAX_VALUE;
        int closetSum = 0;
        for(int i=0; i<=num.length-3; i++) {
            int left = i+1;
            int right = num.length-1;
            while(left < right) {
                int sum = num[i]+num[left]+num[right];
                if(Math.abs(sum-target) < diff) {
                    diff = Math.abs(sum-target);
                    closetSum = sum;
                }
                if(sum == target) {
                    return sum;
                } else if(sum < target) {
                    left++;
                } else {
                    right--;
                }
            }
        }
        return closetSum;
    }
}