[letecode java]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

题目首先想到的就是双层循环，作为一道medium题，无疑会超时。所以要借助一个HashMap,这样一次遍历即可，时间复杂度为o(n)，空间复杂度为o(n)。代码如下：

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