Two Sum (Java)

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最新推荐文章于 2023-08-21 04:02:53 发布

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本文介绍了一种解决两数之和问题的有效算法。通过使用哈希表存储数组元素及其索引，可以在O(n)的时间复杂度内找到两个数，使它们的和等于给定的目标值。文章提供了一个Java实现示例。

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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

方法是设一个hashmap，将number值设为key，index设为value。如果没找到target-number就存入number，否则输出。

Source

public class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int[] a = new int[2];
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        if(numbers.length < 2) return a;
        
        for(int i = 0; i < numbers.length; i++){
        	if(map.containsKey(target - numbers[i])){  //如果存在，必小于i,因为从头开始遍历的，添加进去的都是遍历过的
        		a[0] = map.get(target - numbers[i]) + 1; //小于i+1
        		a[1] = i + 1;
        		break;
        	}
        	else{
        		map.put(numbers[i], i);
        	}
        }
        return a;
    }
}

Test

    public static void main(String[] args){
    	int[] numbers = {-1, 0, 1, 2};
    	int[] b = new Solution().twoSum(numbers, 3);
    	System.out.println(b[0]);
    	System.out.println(b[1]);
    }