1. Two Sum

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, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

vector<int> twoSum(vector<int>& nums, int target) { // time: O(n); space: O(n)
    unordered_map<int, int> num2idx; // number -> index
    for (int i = 0; i < nums.size(); ++i) {
        if (num2idx.count(target - nums[i])) {
            return vector<int>({num2idx[target - nums[i]], i});
        }
        num2idx[nums[i]] = i;
    }
    return vector<int>({-1, -1});
}

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 target - nums[i] in dict:
                return [dict[target - nums[i]], i]
            else:
                dict[nums[i]] = i