1. Two Sum

Approach 1: Store value, index pairs

class Solution(object):
def twoSum(self, nums, target):
: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]
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)