【Leetcode】Two Sum (2 Sum)

最新推荐文章于 2025-12-16 18:10:48 发布

转载最新推荐文章于 2025-12-16 18:10:48 发布 · 156 阅读

文章标签：

#leetcode #Java #hashmap #Sum

本文介绍了一种解决两数之和问题的有效算法。给定一个整数数组及目标值，寻找两个数相加等于目标值，并返回这两个数的下标。文章详细解释了使用哈希表实现该算法的过程，其时间复杂度为O(n)，空间复杂度同样为O(n)。

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9

Output: index1=1, index2=2

Java:

http://blog.youkuaiyun.com/linhuanmars/article/details/19711387

1.用哈希表，时间复杂度为O(n),同时空间复杂度也是O(n)

public class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int[] res= new int[2];
        if(numbers==null||numbers.length<2) return null;
        HashMap<Integer, Integer> map=new HashMap<Integer, Integer>();
        for(int i=0;i<numbers.length;i++)
        {
            if(map.containsKey(target-numbers[i]))
            {
                res[0]=map.get(target-numbers[i])+1;
                res[1]=i+1;
                return res;
            }
            else
            {
                map.put(numbers[i],i);
            }
        }
        return null;
    }
}

第二种解法没有完成，还没想到好的数据结构，用hashmap {[0,4,3，0], 0}过不了