1. Two Sum

Approach 1: Store value, index pairs

class Solution(object):
    def twoSum(self, nums, target):
        """
        https://leetcode.com/problems/two-sum/description/
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        di = {}
        for i in range(len(nums)):
            if target - nums[i] in di:
                return [di[target - nums[i]], i]
            else:
                di[nums[i]] = i

For each of the numbers in the array, store their value and index in a hash table, and every time, check if target-nums[i] exists in the hash table already. If yes we can return the indices of both these values.

Time Complexity: O(N), where N = len(nums)

Space Complexity: O(N), where N = len(nums)