Leetcode binary tree algorithm

 

LeetCode-python 101. Symmetric binary tree

 

 

Difficulty: Simple Type: Binary Tree

 

---

 

Given a binary tree, check if it is mirror-symmetric.

 

For example, the binary tree [1,2,2,3,4,4,3] is symmetric.

 

    1
   / \
  2   2
 / \ / \
3  4 4  3

 

But the following [1,2,2, null, 3, null, 3] is not mirror-symmetric:

 

    1
   / \
  2   2
   \   \
   3    3

 

Problem-solving ideas

 

---

 

Turn this question into whether the left and right subtrees of the root node are mirror-symmetric

 

The conditions for the two trees to mirror each other are:

 

1. Their root nodes have the same value
2. The left subtree of each tree is mirror-symmetric to the right subtree of another tree

 

Code

 

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

class Solution:
    def convers(self,p,q):
        if p == None and q == None:
            return True
        elif p == None or q == None:
            return False
        elif p.val == q.val:
            return self.convers(p.left, q.right) and self.convers(p.right, q.left)
        else:
            return False

    def isSymmetric(self, root: TreeNode) -> bool:
        if root:
            return self.convers(root.left,root.right)
        else:
            return True

 

LeetCode-python 104. Maximum depth of binary tree

 

Difficulty: Simple Type: Binary tree, recursive

---

Given a binary tree, find its maximum depth.

The depth of the binary tree is the number of nodes on the longest path from the root node to the furthest leaf node.

Example
given a binary tree [3,9,20, null, null, 15,7]

    3
   / \
  9  20
    /  \
   15   7

Problem-solving ideas

---

The maximum depth is the greater of +1 in the depth of the left or right subtree, recursively looking in the subtree of the subtree

Code

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

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        else:
            l = self.maxDepth(root.left)
            r = self.maxDepth(root.right)
            return max(l,r)+1