Leetcode刷题笔记（一）

654. Maximum Binary Tree

Given an integer array with no duplicates. A maximum tree building on this array is defined as follow:

1. The root is the maximum number in the array.
2. The left subtree is the maximum tree constructed from left part subarray divided by the maximum number.
3. The right subtree is the maximum tree constructed from right part subarray divided by the maximum number.

Construct the maximum tree by the given array and output the root node of this tree.

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给定一个没有重复的整数数组。在此阵列上建立的最大树定义如下：

1. 根是数组中的最大数目。
2. 左边的子树是从左边的子数组中除以最大数目而得到的最大树。
3. 右子树是从右部分子数组构造的最大树除以最大数目。

通过给定的数组构造最大树并输出此树的根节点。

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Example 1:

Input: [3,2,1,6,0,5]
Output: return the tree root node representing the following tree:

      6
    /   
   3     5
        / 
     2  0   
       
        1

Note:

1. The size of the given array will be in the range [1,1000].

题目不算难，可以很容易的想到解决思路：

1. 创建一个根节点，其值为数组的最大值
2. 创建根节点的左节点，左节点的值为数组左半部分的最大值
3. 创建根节点的右节点，右节点的值为数组右半部分的最大值
4. 递归以上操作

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def constructMaximumBinaryTree(self, nums: List[int]) -> TreeNode:
        if nums ==[]:
            return None
        index = nums.index(max(nums))
        root = TreeNode(max(nums))
        root.left = self.constructMaximumBinaryTree(nums[:index])
        root.right = self.constructMaximumBinaryTree(nums[index+1:])
        return root

　　

1305. All Elements in Two Binary Search Trees

 

Given two binary search trees root1 and root2.

Return a list containing all the integers from both trees sorted in ascending order.

Example 1:

Input: root1 = [2,1,4], root2 = [1,0,3]
Output: [0,1,1,2,3,4]

Example 2:

Input: root1 = [0,-10,10], root2 = [5,1,7,0,2]
Output: [-10,0,0,1,2,5,7,10]

Example 3:

Input: root1 = [], root2 = [5,1,7,0,2]
Output: [0,1,2,5,7]

Example 4:

Input: root1 = [0,-10,10], root2 = []
Output: [-10,0,10]

Example 5:

Input: root1 = [1,null,8], root2 = [8,1]
Output: [1,1,8,8]

Constraints:

• Each tree has at most 5000 nodes.
• Each node's value is between [-10^5, 10^5].

516. Longest Palindromic Subsequence

Given a string s, find the longest palindromic subsequence's length in s. You may assume that the maximum length of s is 1000.

Example 1:
Input:

"bbbab"

Output:

4

One possible longest palindromic subsequence is "bbbb".

Example 2:
Input:

"cbbd"

Output:

2

One possible longest palindromic subsequence is "bb".

DP解决

 python

from functools import lru_cache #不加这个可能会超时
class Solution:
    def longestPalindromeSubseq(self, s: str) -> int:
        @lru_cache(None)
        def dp(i, j):
            if i == j:
                return 1
            elif i > j:
                return 0
            if s[i] == s[j]:
                return 2 + dp(i + 1, j - 1)
            else:
                return max(dp(i + 1, j), dp(i, j - 1))
            
        s = list(s)
        return dp(0, len(s) - 1)

1790. Rotate String II

 (LintCode)

Description

Given a string(Given in the way of char array), a right offset and a left offset, rotate the string by offset in place.(left offest represents the offset of a string to the left,right offest represents the offset of a string to the right,the total offset is calculated from the left offset and the right offset,split two strings at the total offset and swap positions)。

Example

Example 1:

Input：str ="abcdefg", left = 3, right = 1
Output："cdefgab"
Explanation：The left offset is 3, the right offset is 1, and the total offset is left 2. Therefore, the original string moves to the left and becomes "cdefg"+ "ab".

Example 2:

Input：str="abcdefg", left = 0, right = 0
Output："abcdefg"
Explanation：The left offset is 0, the right offset is 0, and the total offset is 0. So the string remains unchanged.

Example 3:

Input：str = "abcdefg",left = 1, right = 2
Output："gabcdef"
Explanation：The left offset is 1, the right offset is 2, and the total offset is right 1. Therefore, the original string moves to the left and becomes "g" + "abcdef".

class Solution:
    """
    @param str: A String
    @param left: a left offset
    @param right: a right offset
    @return: return a rotate string
    """
    def RotateString2(self, str, left, right):
  
        diff = (-left + right) % len(str)
        left = str[:-diff]
        right = str[-diff:]
        result = right + left
        
        return result

409. Longest Palindrome

Given a string which consists of lowercase or uppercase letters, find the length of the longest palindromes that can be built with those letters.

This is case sensitive, for example "Aa" is not considered a palindrome here.

Note:
Assume the length of given string will not exceed 1,010.

Example:

Input:
"abccccdd"

Output:
7

Explanation:
One longest palindrome that can be built is "dccaccd", whose length is 7.

 

class Solution:
    def longestPalindrome(self, s: str) -> int:
        k = {}
        result = 0
        center = False
        for i in range(0,len(s)):
            if s[i] in k:
                k[s[i]] = k[s[i]] + 1
            else:
                k[s[i]] = 1
        for value in k.values():
            if value % 2 ==0:

                result += value
            else :
                if center == False:
                    center = True
                    result +=  1
                result += value - 1
        return result   
     

from collections import Counter

class Solution:
    def longestPalindrome(self, s: str) -> int:
        count = sum([(x//2) * 2 for x in Counter(s).values()])
        return count if count == len(s) else (count + 1)

另一种更快的方法