1. Two Sum

时间复杂度O(n)
空间复杂度O(n)

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        dict = {}
        for i in range(len(nums)):
            if nums[i] in dict:
                return [dict[nums[i]], i]
            dict[target-nums[i]] = i
        return None

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