LeetCode解题报告--3 Sum

2024年5月7日 115次阅读

题目： 3 个数和问题
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.

Note:
Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
The solution set must not contain duplicate triplets.
For example, given array S = {-1 0 1 2 -1 -4},

A solution set is:
(-1, 0, 1)
(-1, -1, 2)

原题链接地址：https://leetcode.com/problems/3sum/
分析：题意是从给定的整型数组，找出三个数和为0的所有可能的组合，该题有两种方法解决：
1. 暴力法： 通过三个for循环找出给定数组所有triplet和的情况，并通过if语句判断，将和为0的triplet记录，并打印出来！时间复杂度为0（N^3）,必然TLE，无法accepted。
2. 首尾指针移位法： 先将原数组排序，再设left(l),right(r)（指针）变量分别指向给定数组的首末元素，将3个数和问题转为2个数和2sum问题。时间复杂度O（NlogN）。
以上方法要注意最后的除重操作！
暴力法 java代码（超时无法accepted）

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;
import java.util.Set;

import com.sun.xml.internal.bind.v2.runtime.unmarshaller.XsiNilLoader.Array;
import com.sun.xml.internal.bind.v2.schemagen.xmlschema.List;

public class ThreeSum {

    /** * @param args */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int[] s = new int[] { 0,2,1,-3 };
        System.out.println(" A solution set is: ");
        ArrayList<ArrayList<Integer>> listArray = new ArrayList<ArrayList<Integer>>();
        listArray = threeSum(s);
        for (int i = 0; i < listArray.size(); i++) {
            System.out.println(listArray.get(i));
        }
    }

    public static ArrayList<ArrayList<Integer>> threeSum(int[] nums) {
        ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();
        String string = "";
        ArrayList<String> string1 =new ArrayList<String>();
        int[] b = new int[3];
        for(int i = 0;i < nums.length;i ++){
            for(int j = i + 1;j < nums.length;j ++){
                for(int k = j + 1;k < nums.length;k ++){
                    ArrayList<Integer> element = new ArrayList<Integer>();
                    if(nums[k] + nums[j] + nums[i] == 0){
                        string  = "";
                        element.add(nums[i]);
                        element.add(nums[j]);
                        element.add(nums[k]);

                        b[0] = nums[i];
                        b[1] = nums[j];
                        b[2] = nums[k];
                        //Avoid reduplicated result
                        Arrays.sort(b);
                        for(int n = 0;n < b.length;n ++){
                            string = string + b[n];
                        }
                        if(!string1.contains(string)){
                            list.add(element);
                            string1.add(string);
                        }
                    }
                }
            }
        }
        return list;
    }
}

首尾指针移位法 java代码（accepted）

    public static ArrayList<ArrayList<Integer>> threeSum(int[] nums) {
        ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();
        HashSet<ArrayList<Integer>> elementSet = new HashSet<ArrayList<Integer>>();
        int sum = 0;
        Arrays.sort(nums);
        for (int i = 0; i < nums.length - 2; i++) {
            //Directly avoid reduplicated result 
            int left = i + 1;
            int right = nums.length - 1;
            while (left < right) {
                sum = nums[i] + nums[left] + nums[right];
                if(sum == 0){
                    ArrayList<Integer> element = new ArrayList<Integer>();
                    element.add(nums[i]);
                    element.add(nums[left]);
                    element.add(nums[right]);
                    if(!elementSet.contains(element)){
                        list.add(element);
                        elementSet.add(element);
                    }
                    left ++;
                    right --;
                }else if(sum < 0){
                    left ++;
                }else{
                    right --;
                }
            }
        }
        return list;
    }
}

测试结果：

 A solution set is: 
[-15, 1, 14]
[-15, 2, 13]
[-15, 3, 12]
[-15, 4, 11]
[-15, 5, 10]
[-15, 6, 9]
[-15, 7, 8]
[-14, 0, 14]
[-14, 1, 13]
[-14, 2, 12]
[-14, 3, 11]
[-14, 4, 10]
[-14, 5, 9]
[-14, 6, 8]
[-14, 7, 7]
[-13, -1, 14]
[-13, 0, 13]
[-13, 1, 12]
[-13, 2, 11]
[-13, 3, 10]
[-13, 4, 9]
[-13, 5, 8]
[-13, 6, 7]
[-12, -2, 14]
[-12, -1, 13]
[-12, 0, 12]
[-12, 1, 11]
[-12, 2, 10]
[-12, 3, 9]
[-12, 4, 8]
[-12, 5, 7]
[-12, 6, 6]
[-11, -3, 14]
[-11, -2, 13]
[-11, -1, 12]
[-11, 0, 11]
[-11, 1, 10]
[-11, 2, 9]
[-11, 3, 8]
[-11, 4, 7]
[-11, 5, 6]
[-10, -4, 14]
[-10, -3, 13]
[-10, -2, 12]
[-10, -1, 11]
[-10, 0, 10]
[-10, 1, 9]
[-10, 2, 8]
[-10, 3, 7]
[-10, 4, 6]
[-10, 5, 5]
[-9, -5, 14]
[-9, -4, 13]
[-9, -3, 12]
[-9, -2, 11]
[-9, -1, 10]
[-9, 0, 9]
[-9, 1, 8]
[-9, 2, 7]
[-9, 3, 6]
[-9, 4, 5]
[-8, -6, 14]
[-8, -5, 13]
[-8, -4, 12]
[-8, -3, 11]
[-8, -2, 10]
[-8, -1, 9]
[-8, 0, 8]
[-8, 1, 7]
[-8, 2, 6]
[-8, 3, 5]
[-8, 4, 4]
[-7, -7, 14]
[-7, -6, 13]
[-7, -5, 12]
[-7, -4, 11]
[-7, -3, 10]
[-7, -2, 9]
[-7, -1, 8]
[-7, 0, 7]
[-7, 1, 6]
[-7, 2, 5]
[-7, 3, 4]
[-6, -6, 12]
[-6, -5, 11]
[-6, -4, 10]
[-6, -3, 9]
[-6, -2, 8]
[-6, -1, 7]
[-6, 0, 6]
[-6, 1, 5]
[-6, 2, 4]
[-6, 3, 3]
[-5, -5, 10]
[-5, -4, 9]
[-5, -3, 8]
[-5, -2, 7]
[-5, -1, 6]
[-5, 0, 5]
[-5, 1, 4]
[-5, 2, 3]
[-4, -4, 8]
[-4, -3, 7]
[-4, -2, 6]
[-4, -1, 5]
[-4, 0, 4]
[-4, 1, 3]
[-4, 2, 2]
[-3, -2, 5]
[-3, -1, 4]
[-3, 0, 3]
[-3, 1, 2]
[-2, -2, 4]
[-2, -1, 3]
[-2, 0, 2]
[-2, 1, 1]
[-1, -1, 2]
[-1, 0, 1]
[0, 0, 0]

相关代码放在个人github:https://github.com/gannyee/LeetCode/tree/master/src

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