【leetcode37-51】二叉树

94. 二叉树的中序遍历

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        if not root:
            return []
        left = self.inorderTraversal(root.left)
        right = self.inorderTraversal(root.right)
        return left + [root.val] + right

104.二叉树的最大深度

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        if not root:
            return 0
        return 1+max(self.maxDepth(root.left), self.maxDepth(root.right))

226.翻转二叉树

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        if not root: 
            return 
        root.left, root.right = root.right, root.left
        self.invertTree(root.left)
        self.invertTree(root.right)
        return root

101.对称二叉树

【递归法】
class Solution:
    def isSymmetric(self, root: TreeNode) -> bool:
        if not root:
            return True
        return self.compare(root.left, root.right)
        
    def compare(self, left, right):
        #首先排除空节点的情况
        if left == None and right != None: return False
        elif left != None and right == None: return False
        elif left == None and right == None: return True
        #排除了空节点，再排除数值不相同的情况
        elif left.val != right.val: return False
        
        #此时就是：左右节点都不为空，且数值相同的情况
        #此时才做递归，做下一层的判断
        outside = self.compare(left.left, right.right) #左子树：左、 右子树：右
        inside = self.compare(left.right, right.left) #左子树：右、 右子树：左
        isSame = outside and inside #左子树：中、 右子树：中 （逻辑处理）
        return isSame

543.二叉树的直径

二叉树的直径，因为他不一定经过root，但是，最长路径，一定是有个公共祖先，所以，只要有个全局变量self.max记录就行

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def __init__(self):
        self.max = 0
    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
        self.compute(root)
        return self.max

    def compute(self, root):
        if not root: return 0
        left = self.compute(root.left)
        right = self.compute(root.right)
        self.max = max(self.max, left+right)
        return max(left, right)+1

102.二叉树的层序遍历【没看明白】

class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        levels = []
        self.helper(root, 0, levels)
        return levels
    
    def helper(self, node, level, levels):
        if not node:
            return
        if len(levels) == level:  #先按层数，把【】里面套的【】全都建好
            levels.append([])

        levels[level].append(node.val)
        self.helper(node.left, level + 1, levels)  #指定框子
        self.helper(node.right, level + 1, levels)

108.将有序数组转换为二叉搜索树