二叉树

二叉树包含节点：一个节点包含节点值及子节点

class TreeNode:
    def __init__(self,val =0, left = None, right = None):
        self.val = val
        self.left = left
        self.right = right

# 给二叉树的根节点,返回节点值的前序遍历
class Solution:

    # 前序遍历(根-左-右)
    def preorderTraversal(self, root: TreeNode) -> list:
        list = []
        def per(root):
            if not root:
                return
            list.append(root.val)
            per(root.left)
            per(root.right)
        per(root)
        return list

    # 中序遍历(左-根-右)
    def preorder(self, root: TreeNode) -> list:
        list = []
        def per(root):
            if not root:
                return
            per(root.left)
            list.append(root.val)
            per(root.right)
        per(root)
        return list

    # 后序遍历(左-根-右)
    def preorder_right(self, root: TreeNode) -> list:
        list = []
        def per(root):
            if not root:
                return
            per(root.left)
            per(root.right)
            list.append(root.val)

        per(root)
        return list

    # 求二叉树的深度
    def maxDepth(self, root: TreeNode):
        if root is None: # 递归的终止条件:最后一个节点的子节点深度为0
            return 0
        else:
            # left_height = self.maxDepth(root.left)
            # right_height = self.maxDepth(root.right)
            # return max(left_height, right_height) + 1
            return max(self.maxDepth(root.left),self.maxDepth(root.right))+1 # 每一个节点的深度都等于他的子节点+1






s = Solution()
r1 = TreeNode(1)
r2 = TreeNode(2)
r3 = TreeNode(3)
r4 = TreeNode(4)
r1.left = r2
r1.right = r4
r2.left = r3

# s.preorderTraversal(r1)
print(s.maxDepth(r1))