16. 3Sum Closest

一,题目

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).

二,思路

方法一:枚举+剪枝

方法二:two pointer

方法二比方法一了一层循环

三,代码

public  int threeSumClosest(int[] nums, int target) {
		if (nums == null || nums.length < 3) {
			return 0;
		}

		int len = nums.length;
		int diff = Integer.MAX_VALUE;
		int sum = 0;
		Arrays.sort(nums);

		for (int i = 0; i < len - 2; i++) {
			for (int j = i + 1; j < len - 1; j++) {
				int right = j+1;
				while (right < len) {
					int tempSum = nums[i] + nums[j] + nums[right];
					int tempDiff = tempSum - target;
					int abs = Math.abs(tempDiff);
					if (abs < diff) {
						sum = tempSum;
						diff = abs;
					}
					 if(tempDiff > 0 && tempDiff > diff){
						 break;
					 }
					
					right++;
				}
			}
		}
		return sum;
	}

	public  int threeSumClosest2(int[] nums, int target){
		if (nums == null || nums.length < 3) {
			return 0;
		}

		Arrays.sort(nums);
		int len = nums.length;
		int bestSum = nums[0]+nums[1]+nums[2];
		for(int i=0; i<len-2 ;i++){
			int left = i+1;
			int right = len -1;
			while(left < right){
				int sum = nums[i]+nums[left]+nums[right];
				if(Math.abs(sum - target) < Math.abs(bestSum-target)){
					bestSum = sum;
				}
				if(sum > target){
					right --;
				}else{
					left++;
				}
			}
		}
		
		return bestSum;
	}