[leedcode 01] 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

此题是在一组数组中找到给定值的和的下标。

1.可以利用java排序算法，然后声明两个指针，一头一尾，如果两个指针指向的和大于target，尾指针--，反之，头指针++；时间复杂度O（nlogn）；

2.使用hashmap，空间换时间，时间复杂度是O（n）。key保存target-nums[i],value保存下标。然后遍历一遍数组即可。

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