给定 nums = [2, 7, 11, 15], target = 9
因为 nums[0] + nums[1] = 2 + 7 = 9
所以返回 [0, 1]

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

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