1. 两数之和

class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap map = new HashMap(16);
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(nums[i])){
                return new int[]{i, (int) map.get(nums[i])};
            }
            map.put(target-nums[i],i);
        }
        return nums;
    }
}