Two Sum Java

最新推荐文章于 2021-03-20 03:34:06 发布

Spencer_Lin

最新推荐文章于 2021-03-20 03:34:06 发布

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分类专栏： LeetCode String+Array 文章标签： leetcode array

本文链接：https://blog.youkuaiyun.com/linxiang0591/article/details/38706661

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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目标值减去当前元素值得到的差值及其索引，可以在遍历数组的同时查找是否存在匹配的元素，从而快速找到两个数的索引。这种方法的时间复杂度为O(n)，适用于面试和技术分享。

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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

Better Approach:
Key to solve: HashMap AC 432ms
Trade off between time and space,
Use Map(key: number, value: index)
number1+number2=target => target-number1=number2;
Time: O(n)

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