[leetcode]--1. Two Sum-优快云博客

本文介绍了一种解决两数之和问题的有效算法。通过使用哈希表来存储数组元素及其索引，可以在O(n)时间内找到两个数，使得它们的和等于目标值。这种方法避免了传统双层循环的时间复杂度过高的问题。

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:
Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

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

public class Main {
    //大体思路是空间换时间解决时间复杂度是n平方的问题
    public static int[] twoSum(int[] nums,int target){
        Map<Integer,Integer> map = new HashMap<>();
        for(int i= 0;i<nums.length;i++){//注意这里不要用等号，否者数组越界
            map.put(nums[i],i);
        }
        for(int i= 0;i<nums.length;i++){
            int complement = target - nums[i];
            if(map.containsKey(complement)&& map.get(complement)!= i){
                return  new int[] {i,map.get(complement)};
            }
        }
        throw new  IllegalArgumentException("No two sum solution");

    }
    public static void main(String[] args) {
        int [] arr=new int [ ]{1,2,3,4};
        int [] result;
        result = twoSum(arr,5);
        for(int j=0;j<result.length;j++){
            System.out.println(result[j]);
        }
    }
}