1. Two Sum

题目描述：Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution.

Example:

Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Solution One：使用两重循环解决：

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] result = {0,1};
         for (int i = 0; i < nums.length-1; i++) {
             for (int j = i+1; j < nums.length; j++) {
                 if (nums[i] + nums[j]==target) {
                     result[0]=i;
                     result[1]=j;
                     return result;
                 }
             }
         }
         return result;
    }
}

Solution Two：使用hashmap，把数组装入map中，把数组值定为key，小标定为value，然后对于数组中的每一个key，遍历去寻找其他key，满足相加可以等于target：

   public int[] twoSum(int[] numbers, int target) {
        Map<Integer,Integer> map = new HashMap<Integer, Integer>();
        for (int i=0;i<numbers.length;i++) {
            map.put(numbers[i], i);
        }
        for (int i=0;i<numbers.length;i++) {
            int dif = target-numbers[i];
            Integer index = map.get(dif);
              
            if (index!=null && i!=index) {
                if (index>i)
                    return new int[]{i,index};
                else
                    return new int[]{index,i};
            }
        }
          
        return null;
    }
}