LeetCode 1 与 202

一、 LeetCode 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].

• 思路：
  把所有数存入哈希表，将target与每个数的差值作为key，通过查表找到另一个数。
• 代码如下：

class Solution:
    
    def twoSum(self, nums, target):
        \"\"\"
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        \"\"\"
        hash_table = {nums[k]:k for k in range(len(nums))}  # 为nums创建{value: index}逆向哈希表
        for i in range(len(nums)):
            # 找到第i个数和其他某个数之和等于target的数
            key = target-nums[i]  
            if key in hash_table and i != hash_table[key]:
                return [i, hash_table[key]]

二、 LeetCode 202 Happy Number

Write an algorithm to determine if a number is “happy”.

A happy number is a number defined by the following process: Starting with >any positive integer, replace the number by the sum of the squares of its >digits, and repeat the process until the number equals 1 (where it will >stay), or it loops endlessly in a cycle which does not include 1. Those >numbers for which this process ends in 1 are happy numbers.

Example:

Input: 19
Output: true
Explanation:
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

• 思路：
  把遍历过的数存入哈希表，如果重复则表示链路有环，不为快乐数。
• 代码如下：

class Solution:
    def isHappy(self, n):
        \"\"\"
        :type n: int
        :rtype: bool
        \"\"\"
        hash_set = {n}
        while True:
            n = sum(map(lambda x:x**2, map(int, str(n))))
            if n == 1:
                return True
            if n in hash_set:
                return False
            else:
                hash_set.add(n)