refer to https://blog.csdn.net/linfeng886/article/details/79772348

Description

 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].

Python code

1. Brute Force

class Solution:
def twoSum(self, nums: List[int], target: int) -> List[int]:
N = len(nums)
for i in range(N):
for j in range(i+1, N): # Traverse the next element in turn
if nums[j] == target - nums[i]:
return [i,j]
break
else:
continue

This methods have high computation complexity and memory storage.

2. one for loop

class Solution:
def twoSum(self, nums: List[int], target: int) -> List[int]:
N = len(nums)
for i in range(N):
rest = target - nums[i]
if rest in nums:
j = nums.index(rest)
return [i,j]
break
else:
continue

3. create a dict first

class Solution:
def twoSum(self, nums: List[int], target: int) -> List[int]:
N = len(nums)
d = {}
for i in range(N):
rest = target - nums[i]
if nums[i] in d:
return d[nums[i]], i
else:
d[rest] = i

												

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