Easy Collections
1 Array
Remove Duplicates from Sorted Array
https://leetcode.com/explore/interview/card/top-interview-questions-easy/92/array/727/
Two pointer
Rotate Array
https://leetcode.com/explore/interview/card/top-interview-questions-easy/92/array/646/
k % len(nums) 三次翻转
Valid Sudoku
https://leetcode.com/problems/valid-sudoku/discuss/15460/1-7-lines-Python-4-solutions
enumerate()
seen += [(i, c), (c, j), (i//3, j//3, c)]
通过把row和column index位置放置在不同的位置，统一比较的逻辑
list comprehension https://blog.csdn.net/zhanh1218/article/details/26590837
Rotate Image
https://leetcode.com/problems/rotate-image/discuss/18884/Seven-Short-Solutions-(1-to-7-lines)
https://stackoverflow.com/questions/509211/understanding-slice-notation
matrix[:] = zip(*(matrix[::-1]))
使用zip和unzip
左向翻转
https://www.runoob.com/python/python-func-map.html
matrix = [[1,2,3],[4,5,6],[7,8,9]]
matrix[:] = map(lambda x: x[::-1], matrix[:])
print(matrix)
matrix[:] = map(list, zip(*(matrix[:])))
print(matrix)
2 String
Reverse Integer
https://leetcode.com/problems/reverse-integer/discuss/4055/Golfing-in-Python
The range of a 32-bit signed integer would be-231 ~ 231-1, which is -2147483648 ~ 2147483647. So if I input -8463847412 (the reverse integer is exactly -2**31, inside the range)，because -8463847412 is not a valid 32 bit int
class Solution(object):
def reverse(self, x):
s = (x > 0) - (x < 0)
r = int(str(x*s)[::-1])
return s*r * (r < 2**31)
First Unique Character in a String
https://leetcode.com/explore/interview/card/top-interview-questions-easy/127/strings/881/
使用python counter类（dict的子类）
Valid Palindrome
https://leetcode.com/problems/valid-palindrome/discuss/39982/Python-in-place-two-pointer-solution.
class Solution:
def isPalindrome(self, s):
s = ''.join(e for e in s if e.isalnum()).lower()
return s==s[::-1]
https://www.cnblogs.com/ilyou2049/p/11123299.html
isalnum()方法 isalpha()方法 isdigit()方法
String to Integer
https://leetcode.com/problems/string-to-integer-atoi/discuss/4673/60ms-python-solution-OJ-says-this-beats-100-python-submissions
class Solution(object):
def myAtoi(self, s):
"""
:type str: str
:rtype: int
"""
ls = list(s.strip())
if len(ls) == 0 : return 0
sign = -1 if ls[0] == '-' else 1
if ls[0] in ['-','+'] : del ls[0]
ret, i = 0, 0
while i < len(ls) and ls[i].isdigit() :
ret = ret*10 + ord(ls[i]) - ord('0')
i += 1
return max(-2**31, min(sign * ret,2**31-1))
Count and Say
https://leetcode.com/problems/count-and-say/discuss/16044/Simple-Python-Solution
注意是n-1和+=
def countAndSay(self, n):
s = '1'
for _ in range(n-1):
let, temp, count = s[0], '', 0
for l in s:
if let == l:
count += 1
else:
temp += str(count)+let
let = l
count = 1
temp += str(count)+let
s = temp
return s
Longest Common Prefix
https://leetcode.com/problems/longest-common-prefix/discuss/6918/Short-Python-Solution
注意最后return shortest
def longestCommonPrefix(self, strs):
"""
:type strs: List[str]
:rtype: str
"""
if not strs:
return ""
shortest = min(strs,key=len)
for i, ch in enumerate(shortest):
for other in strs:
if other[i] != ch:
return shortest[:i]
return shortest
3 LinkedList
Remove Nth Node From End of List
https://leetcode.com/explore/interview/card/top-interview-questions-easy/93/linked-list/603/
可以使用双指针来计算
Reverse Linked List
https://leetcode.com/explore/interview/card/top-interview-questions-easy/93/linked-list/560/
也可以考虑用递归的方法做
# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if not head or not head.next:
return head
p = self.reverseList(head.next)
head.next.next = head
head.next = None
return p
Palindrome Linked List
https://leetcode.com/explore/interview/card/top-interview-questions-easy/93/linked-list/772/
https://leetcode.com/problems/palindrome-linked-list/discuss/64514/5-lines-Python-O(n)-time-and-space
不考虑in place直接翻转比较
def isPalindrome(self, head):
vals = []
while head:
vals += head.val,
head = head.next
return vals == vals[::-1]
考虑in place 用快慢指针，翻转后一半，比较前后两版
https://leetcode.com/problems/palindrome-linked-list/discuss/64689/Python-easy-to-understand-solution-with-comments-(operate-nodes-directly).
Linked List Cycle
https://leetcode.com/explore/featured/card/top-interview-questions-easy/93/linked-list/773/
用dict或者two pointer
4 Trees
Validate Binary Search Tree
https://leetcode.com/explore/featured/card/top-interview-questions-easy/94/trees/625/
iteration或者recursion或者inorder traversal
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def isValidBST(self, root):
"""
:type root: TreeNode
:rtype: bool
"""
def helper(root, lower=float("-inf"), upper=float("inf")):
if not root:
return True
if not helper(root.left, lower, root.val):
return False
if not helper(root.right, root.val, upper):
return False
if not (lower < root.val < upper):
return False
return True
return helper(root)
Symmetric Tree
https://leetcode.com/explore/interview/card/top-interview-questions-easy/94/trees/627/
使用两个root，t1, t2
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def isSymmetric(self, root):
"""
:type root: TreeNode
:rtype: bool
"""
def isMirror(t1, t2):
if t1 == None and t2 == None:
return True
if t1 == None or t2 == None:
return False
return t1.val == t2.val and isMirror(t1.left, t2.right) and isMirror(t1.right, t2.left)
return isMirror(root, root)
Binary Tree Level Order Traversal
https://leetcode.com/explore/interview/card/top-interview-questions-easy/94/trees/628/
deque, queue, level, node
Convert Sorted Array to Binary Search Tree
https://leetcode.com/explore/interview/card/top-interview-questions-easy/94/trees/631/
class Solution(object):
def sortedArrayToBST(self, nums):
"""
:type nums: List[int]
:rtype: TreeNode
"""
if not nums:
return None
mid = len(nums) // 2
node = TreeNode(nums[mid])
node.left = self.sortedArrayToBST(nums[:mid])
node.right = self.sortedArrayToBST(nums[mid+1:])
return node
5 Sorting and Searching
Merge Sorted Array
https://leetcode.com/problems/merge-sorted-array/discuss/29503/Beautiful-Python-Solution
从后往前写，融合
First Bad Version
https://leetcode.com/explore/interview/card/top-interview-questions-easy/96/sorting-and-searching/774/
https://www.jiuzhang.com/solution/first-bad-version/#tag-highlight-lang-python
相邻即退出
class Solution:
"""
@param n: An integers.
@return: An integer which is the first bad version.
"""
def findFirstBadVersion(self, n):
start, end = 1, n
while start + 1 < end:
mid = (start + end) // 2
if SVNRepo.isBadVersion(mid):
end = mid
else:
start = mid
if SVNRepo.isBadVersion(start):
return start
return end
6 Dynamic Programming
Climbing Stairs
https://leetcode.com/explore/interview/card/top-interview-questions-easy/97/dynamic-programming/569/
从下到上，使用DP
class Solution(object):
def climbStairs(self, n):
"""
:type n: int
:rtype: int
"""
if n == 1:
return 1
dp = [0 for i in range(n+1)]
dp[1] = 1
dp[2] = 2
for i in range(3, n+1):
dp[i] = dp[i-1] + dp[i-2]
return dp[n]
Best Time to Buy and Sell Stock
https://leetcode.com/problems/best-time-to-buy-and-sell-stock/discuss/39049/Easy-O(n)-Python-solution
max_profit和min_price临时存
def maxProfit(prices):
max_profit, min_price = 0, float('inf')
for price in prices:
min_price = min(min_price, price)
profit = price - min_price
max_profit = max(max_profit, profit)
return max_profit
Maximum Subarray
https://leetcode.com/problems/maximum-subarray/discuss/20396/Easy-Python-Way
https://leetcode.com/problems/maximum-subarray/discuss/20194/A-Python-solution
从求和的角度看
class Solution(object):
def maxSubArray(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
if not nums:
return 0
if len(nums) == 1:
return nums[0]
for i in range(1, len(nums)):
if nums[i-1] > 0:
nums[i] += nums[i-1]
return max(nums)
House Robber
DP问题的一般解法：
Find recursive relation
Recursive (top-down)
Recursive + memo (top-down)
class Solution(object):
def rob(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
memo = [-1 for i in range(len(nums))]
return self.helper(nums, len(nums) - 1, memo)
def helper(self, nums, i, memo):
if i < 0:
return 0
if memo[i] >= 0:
return memo[i]
result = max(self.helper(nums, i - 2, memo) + nums[i], self.helper(nums, i - 1, memo))
memo[i] = result
return result
Iterative + memo (bottom-up)
class Solution(object):
def rob(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
if not nums:
return 0
memo = [-1 for i in range(len(nums)+1)]
memo[0] = 0
memo[1] = nums[0]
for i in range(1, len(nums)):
memo[i+1] = max(memo[i], memo[i-1]+nums[i])
return memo[len(nums)]
Iterative + N variables (bottom-up)
https://leetcode.com/problems/house-robber/discuss/156523/From-good-to-great.-How-to-approach-most-of-DP-problems.
7 Design
Shuffle an Array
https://leetcode.com/explore/featured/card/top-interview-questions-easy/98/design/670/
class Solution:
def __init__(self, nums):
self.array = nums
self.original = list(nums)
def reset(self):
self.array = self.original
self.original = list(self.original)
return self.array
def shuffle(self):
for i in range(len(self.array)):
swap_idx = random.randrange(i, len(self.array))
self.array[i], self.array[swap_idx] = self.array[swap_idx], self.array[i]
return self.array
Min Stack
https://leetcode.com/problems/min-stack/discuss/49183/Python-one-stack-solution-without-linklist
使用[(x, cur_min),()]格式来存储，本质上是一个栈
8 Math
Count Primes
https://leetcode.com/explore/interview/card/top-interview-questions-easy/102/math/744/
https://leetcode.com/problems/count-primes/discuss/153528/Python3-99-112-ms-Explained%3A-The-Sieve-of-Eratosthenes-with-optimizations
使用根号n来计算更快，n^2会超时，用每个乘法来算，原理见下：
https://blog.csdn.net/Avan_Lau/article/details/7257566
class Solution(object):
def countPrimes(self, n):
"""
:type n: int
:rtype: int
"""
if n < 2:
return 0
strikes = [1] * n
strikes[0] = 0
strikes[1] = 0
for i in range(2, int(n**0.5) + 1):
if strikes[i] != 0:
#strikes[i*i:n:i] = [0]*((n-1-i*i)//i+1)
index = i*i
while index < n:
strikes[index] = 0
index += i
return sum(strikes)
Power of Three
https://leetcode.com/explore/interview/card/top-interview-questions-easy/102/math/745/
https://leetcode.com/problems/power-of-three/discuss/77977/Math-1-liner-no-log-with-explanation
直接找最大的那个算
def isPowerOfThree(self, n):
return n > 0 == 3**19 % n
Roman to Integer
https://leetcode.com/problems/roman-to-integer/discuss/6537/My-Straightforward-Python-Solution
直接把每个罗马字符对应的数弄成dict
9 Others
Number of 1 Bits
https://leetcode.com/problems/number-of-1-bits/discuss/55106/Python-2-solutions.-One-naive-solution-with-built-in-functions.-One-trick-with-bit-operation
Hamming Distance
https://leetcode.com/problems/number-of-1-bits/discuss/55106/Python-2-solutions.-One-naive-solution-with-built-in-functions.-One-trick-with-bit-operation
def hammingWeight(self, n):
"""
:type n: int
:rtype: int
"""
return bin(n).count('1')
def hammingWeight(self, n):
"""
:type n: int
:rtype: int
"""
c = 0
while n:
n &= n - 1
c += 1
return c
Reverse Bits
https://leetcode.com/problems/reverse-bits/discuss/54748/AC-Python-44-ms-solution-bit-manipulation
class Solution:
# @param n, an integer
# @return an integer
def reverseBits(self, n):
result = 0
for i in range(32):
result = (result << 1) + (n & 1)
n >>= 1
return result
Valid Parentheses
https://leetcode.com/explore/interview/card/top-interview-questions-easy/99/others/721/
Missing Number
https://leetcode.com/explore/interview/card/top-interview-questions-easy/99/others/722/
异或的使用
class Solution(object):
def missingNumber(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
missing = len(nums)
for i, num in enumerate(nums):
missing ^= i ^ num
return missing