#1 Two Sum 兩數之和

just for the record, not the best solutions.

if you like to study together, my discord: Journey#9805

package com.journey;

import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class TowSum_兩數之和_001 {

    //hash
    public static int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(target - nums[i])) {
                result[1] = i;
                result[0] = map.get(target - nums[i]);
                return result;
            }
            map.put(nums[i], i);
        }
        return result;
    }

    //sort + 2 pointers
    public static int[] twoSumSolution2(int[] nums, int target) {
        int m = 0, n = 0, k, board = 0;
        int len = nums.length;
        int[] result = new int[2];
        int[] temp1 = new int[len];
        System.arraycopy(nums, 0, temp1, 0, len);
        Arrays.sort(nums);

        for (int i = 0, j = len - 1; i < j; ) {
            //數組排序後， i = 頭; j = 尾，如果i+j小於target，i後移1位
            if (nums[i] + nums[j] < target) {  
                i++;
            } else if (nums[i] + nums[j] == target) {
                m = i;
                n = j;
                break;
            }
        }
        for (k = 0; k < len; k++) {
            if (temp1[k] == nums[m]) { //對比
                result[0] = k;
                break;
            }
        }
        for (int i = 0; i < len; i++) {
            if (temp1[i] == nums[n] && i != k) {
                result[1] = i;
            }
        }
        return result;
    }

    //for
    public static int[] twoSumSolution3(int[] nums, int target) {
        int[] result = new int[2];
        int len = nums.length;
        int i, j;
        for (i = 0; i < len; i++) {
            for (j = i + 1; j < len; j++) {
                if (nums[i] + nums[j] == target) {
                    result[0] = i;
                    result[1] = j;
                    return result;
                }
            }
        }
        return result;
    }


    public static void main(String[] args) {

        int[] test1 = new int[]{7, 0, 3, 1, 2, 3, 4, 5, 6};
        int targetTest1 = 10;
        
        twoSum(test1,targetTest1);          //  0 2 
        twoSumSolution2(test1,targetTest1); //  2 0
        twoSumSolution3(test1,targetTest1); //  0 2
    }
    
}

Two Sum - LeetCode

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