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

 

最早我使用的方法是两个for循环的方法，时间复杂度为O(n2)。可是，在线测试提示时间超时，所以必须考虑更加简单的方法，

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