LeetCode OJ - 3Sum-优快云博客

本文链接：https://blog.youkuaiyun.com/cinrry/article/details/50497111

本文探讨了在给定整数数组中寻找三个元素之和为零的所有唯一组合的问题，并提供了一个初始解决方案及其优化版本。优化后的算法通过排序和双指针技术显著提高了查找效率。

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Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

my answer:

import java.util.*;

public class Solution {
    public static List<List<Integer>> threeSum(int[] nums) {
    	List<Integer> list_in = new ArrayList<Integer>();
    	List<List<Integer>> list_out = new ArrayList<List<Integer>>();
    	
    	if(nums == null || nums.length<3)
    		return list_out;

    	Arrays.sort(nums);
<span style="white-space:pre">	</span>for(int j=i+1;j<nums.length;j++){
    		if(nums[i]+nums[j]>0 && nums[j]>0) break;
    		for(int k=j+1;k<nums.length;k++){
    			if(nums[i]+nums[j]+nums[k]==0){
    					
    				list_in.add(nums[i]);
    				list_in.add(nums[j]);
    				list_in.add(nums[k]);
    				if(!list_out.contains(list_in))
    					list_out.add(list_in);
    				list_in=new ArrayList<Integer>();			
    			}

    		}
    	}
    	return list_out;
    }

    public static void main(String[] args){
    	List<List<Integer>> list =threeSum(new int[]{-1,0,1,2,-1,-4});
    	//List<List<Integer>> list =threeSum(new int[]{0,0,0,0,0});
    	//List<List<Integer>> list =threeSum(new int[]{0,0,0,0,0});
    	//List<List<Integer>> list =threeSum(new int[]{-4,-2,-2,-2,0,1,2,2,2,3,3,4,4,6,6});
    	//List<List<Integer>> list =threeSum(new int[]{-15,6,7,0,-14,-5,-3,-10,-14,1,-14,-1,-11,-11,-15,-1,3,-12,7,14,1,6,-6,7,1,1,0,-4,8,7,2,1,-2,-6,-14,-9,-3,-1,-12,-2,7,11,4,12,-14,-4,-4,4,-1,10,3,-14,1,12,0,10,-9,8,-9,14,-8,8,0,-3,10,-6,4,-8,0,-1,-3,-8,-4,8,11,-3,-11,-8,8,3,10,-3,-4,-4,-14,12,13,-8,-3,12,-8,4,5,-1,-14,-8,8,-3,-9,-15,12,-5,-7,-15,-12,11,-11,14,11,12,3,6,-6});
    	System.out.println(list);
    }
}

唔，正如我所料，刚开始还是对的，后来数据一多果然超时了，毕竟3个forO(n^3)呐!!!

test到最后[7,-1,14,-12,-8,7,2,-15,8,8,-8,-14,-4,-5,7,9,11,-4,-15,-6,1,-14,4,3,10,-5,2,1,6,11,2,-2,-5,-7,-6,2,-15,11,-6,8,-4,2,1,-1,4,-6,-15,1,5,-15,10,14,9,-8,-6,4,-6,11,12,-15,7,-1,-9,9,-1,0,-4,-1,-12,-2,14,-9,7,0,-3,-4,1,-2,12,14,-10,0,5,14,-1,14,3,8,10,-8,8,-5,-2,6,-11,12,13,-7,-12,8,6,-13,14,-2,-5,-11,1,3,-6]

果不其然的挂掉了...嘤嘤嘤

参考：http://www.programcreek.com/2012/12/leetcode-3sum/

改进：

import java.util.*;

public class Solution_15 {
    public static List<List<Integer>> threeSum(int[] nums) {
    	List<Integer> list_in = new ArrayList<Integer>();
    	List<List<Integer>> list_out = new ArrayList<List<Integer>>();
    	
    	if(nums == null || nums.length<3)
    		return list_out;

    	Arrays.sort(nums);

    	for (int i=0;i<nums.length-2;i++){
    		if(nums[0]>0)
    			break;
    		if(i>1 && nums[i] == nums[i-1])
    			continue;
    		int l = i+1;
    		int r = nums.length-1;
    		int sum = 0;
    		while(l<r){
    			sum = nums[i]+nums[l]+nums[r];
    			if (sum==0){
    				list_in.add(nums[i]);
    				list_in.add(nums[l]);
    				list_in.add(nums[r]);
    				if(!list_out.contains(list_in))
    					list_out.add(list_in);
    				list_in=new ArrayList<Integer>();
    				l++;r--;
    			} else if(sum>0)
    				r--;
    			else if(sum<0)
    				l++;
    		}
    	}
    	return list_out;
    }

    public static void main(String[] args){
    	List<List<Integer>> list =threeSum(new int[]{-1,0,1,2,-1,-4});
    	//List<List<Integer>> list =threeSum(new int[]{0,0,0,0,0});
    	//List<List<Integer>> list =threeSum(new int[]{0,0,0,0,0});
    	//List<List<Integer>> list =threeSum(new int[]{-4,-2,-2,-2,0,1,2,2,2,3,3,4,4,6,6});
    	//List<List<Integer>> list =threeSum(new int[]{-15,6,7,0,-14,-5,-3,-10,-14,1,-14,-1,-11,-11,-15,-1,3,-12,7,14,1,6,-6,7,1,1,0,-4,8,7,2,1,-2,-6,-14,-9,-3,-1,-12,-2,7,11,4,12,-14,-4,-4,4,-1,10,3,-14,1,12,0,10,-9,8,-9,14,-8,8,0,-3,10,-6,4,-8,0,-1,-3,-8,-4,8,11,-3,-11,-8,8,3,10,-3,-4,-4,-14,12,13,-8,-3,12,-8,4,5,-1,-14,-8,8,-3,-9,-15,12,-5,-7,-15,-12,11,-11,14,11,12,3,6,-6});
    	System.out.println(list);
    }
}

Runtime：33ms

然后然后，恩，1.11 星期一，击败了11.11% 嗷嗷~~