【leetcode刷题笔记】Two Sum

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

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利用一个hashmap，把数组中的元素和它们的索引+1作为键-值对，然后对于每个元素numbers[i]寻找target-numbers[i]，如果找到了就把i+1和map.get(target-numbers[i])返回。

代码如下：

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