二叉樹的各種常見題型

前序遍歷

def pre_search(root: TreeNode):
    res = []

    if root is None:
        return res

    stack = [root]  # 使用的是棧
    while stack:
        p = stack.pop()
        res.append(p.val)
        if p.right:  # 先入右子樹
            stack.append(p.right)
        if p.left:  # 後入左子樹
            stack.append(p.left)
    return res

後續遍歷

def post_search(root: TreeNode):
    ans = []
    if root is None:
        return ans

    stack = [root] # 使用的棧
    while stack:
        p = stack.pop()
        ans.append(p.val)

        if p.left: # 先入左子樹
            stack.append(p.left)

        if p.right: # 後入右子樹
            stack.append(p.right)
    return ans.reverse() # 最後翻轉

中序遍歷

def mid_sarch(root: TreeNode):
    ans = []
    if root is None:
        return ans

    stack = []
    node = root
    while stack or node:  # 第一次通過node是否爲空判斷，後面都要通過stack判斷
        # 先把左子樹都入到頭
        while node:
            stack.append(node)
            node = node.left
        # 此時 node 爲空

        data = stack.pop()
        ans.append(data.val)
        node = data.right

    return ans

層序遍歷

def level_search(root: TreeNode):
    ans = []

    if root is None:
        return ans

    queue = [root]  # 層序遍歷使用的是隊列

    while queue:
        top = queue.pop(0)
        ans.append(top.val)
        if top.left:  # 先進左子樹
            queue.append(top.left)

        if top.right:  # 後進右子樹
            queue.append(top.right)
    return ans

一個方法包含所有遍歷方法的非遞歸遍歷

1. 用樹的顏色標記節點狀態
2. 白色代表未遍歷，黑色代表已經遍歷
3. 因爲用的是棧，所以先入右子樹後入左子樹

class Solution:
    def inorderTraversal(self, root: TreeNode):
        if root is None:
            return []
        WHITE, BLACK = 0, 1
        res = []
        stack = [(WHITE, root)]
        while stack:
            color, node = stack.pop()
            if color == WHITE: # 如果是白色的不遍歷，只遍歷黑色的
                if node.right: # 先進右子樹
                    stack.append((WHITE, node.right))
                stack.append((GRAY, node))
                if node.left: # 後進左子樹
                    stack.append((WHITE, node.left))
            else:
                res.append(node.val)
        return res

求樹深

def tree_max_hight(root: TreeNode):
    if root is None:
        return 0

    queue = [(1, root)]
    max_hight = 0

    while queue:
        hight, node = queue.pop(0)
        max_hight = max(max_hight, hight)

        if node.left:
            queue.append((hight + 1, node.left))

        if node.right:
            queue.append((hight + 1, node.right))
    return max_hight

最近公共祖先

def node_father(root, q, p):  # 最近公共祖先
    if root is None:
        return None

    if root == q or root == p:
        return root

    left = node_father(root.left, q, p)
    right = node_father(root.right, q, p)

    if left and right:
        return root

    if left:
        return left

    if right:
        return right

二叉樹翻轉

先翻轉左子樹，再翻轉右子樹
最後把兩棵樹翻轉

def invertTree(root: TreeNode):
    if root is None:
        return root

    root.left = invertTree(root.left)
    root.right = invertTree(root.right)
    root.left, root.right = root.right, root.left

    return root