力扣-两数之和

力扣-两数之和

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

import static java.lang.String.*;

/**
 *力扣：两数之和
 **/
public class TwoNumSum {
    //暴力法
    public static int[] twoSum(int[] nums, int target) {
        if(nums==null||nums.length==0){
            return null;
        }
        for (int i = 0; i < nums.length; i++) {
            for (int j = i+1; j < nums.length; j++) {
                if(nums[i]+nums[j]==target){
                    return new int[]{i,j};
                }
            }
        }
        return null;
    }
    //哈希表
    public static int[] twoSum2(int[] nums, int target) {
        if(nums==null||nums.length==0){
            return null;
        }
        HashMap<Integer, Integer> hashMap = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            hashMap.put(nums[i],i);
        }
        for (int i = 0; i < nums.length; i++) {
            int result=target-nums[i];
            if(hashMap.containsKey(result) && hashMap.get(result)!=i){
                return new int[]{i,hashMap.get(result)};
            }
        }
        return null;
    }
    //哈希表优化
    public static int[] twoSum3(int[] nums, int target) {
        if(nums==null||nums.length==0){
            return null;
        }
        HashMap<Integer, Integer> hashMap = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int x=nums[i];
            if(hashMap.containsKey(target-x)){
                int index=hashMap.get(target-x);
                return new int[]{i,index};
            }
            hashMap.put(nums[i],i);
        }
        return null;
    }

    public static void main(String[] args) {
       int[] nums = {2,7,11,15};
       int target = 9;
        System.out.println(Arrays.toString(twoSum3(nums, target)));
    }

}