hello-algo/codes/python/chapter_computational_complexity/leetcode_two_sum.py

"""
File: leetcode_two_sum.py
Created Time: 2022-11-25
Author: Krahets (krahets@163.com)
"""

import sys, os.path as osp
sys.path.append(osp.dirname(osp.dirname(osp.abspath(__file__))))
from modules import *

def two_sum_brute_force(nums: List[int], target: int) -> List[int]:
    """ 方法一：暴力枚举 """
    # 两层循环，时间复杂度 O(n^2)
    for i in range(len(nums) - 1):
        for j in range(i + 1, len(nums)):
            if nums[i] + nums[j] == target:
                return i, j
    return []

def two_sum_hash_table(nums: List[int], target: int) -> List[int]:
    """ 方法二：辅助哈希表 """
    # 辅助哈希表，空间复杂度 O(n)
    dic = {}
    # 单层循环，时间复杂度 O(n)
    for i in range(len(nums)):
        if target - nums[i] in dic:
            return dic[target - nums[i]], i
        dic[nums[i]] = i
    return []


""" Driver Code """
if __name__ == '__main__':
    # ======= Test Case =======
    nums = [2,7,11,15]
    target = 9
    
    # ====== Driver Code ======
    # 方法一
    res = two_sum_brute_force(nums, target)
    print("方法一 res =", res)
    # 方法二
    res = two_sum_hash_table(nums, target)
    print("方法二 res =", res)