树的各种遍历

树的遍历

1.前序遍历(preoOrder)

https://leetcode.com/problems/binary-tree-preorder-traversal/

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

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res = []
        node = []
        if not root:
            return res
        node.append(root)
        while node:
            temp = node.pop()
            res.append(temp.val)
            if temp.right:
                node.append(temp.right)
            if temp.left:
                node.append(temp.left)
        return res
# 递归实现
# return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right) if root is not None else []

2.中序遍历(inOrder)

https://leetcode.com/problems/binary-tree-inorder-traversal/

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

class Solution(object):
    def inorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        node = []
        res = []
        if not root:
            return res
        node.append(root)
        temp = root
        while node:
            while temp and temp.left:
                node.append(temp.left)
                temp = temp.left
            temp = node.pop()
            res.append(temp.val)
            temp = temp.right
            if temp:
                node.append(temp)
        return res
# 递归实现
#return self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right) if root is not None else []

3.后序遍历(postOrder)

https://leetcode.com/problems/binary-tree-postorder-traversal/

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

class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        # 反的‘前序遍历’(根->右->左)
        node = []
        res = []
        if not root:
            return res
        node.append(root)
        temp = root
        while node:
            temp = node.pop()
            if temp:
                res.append(temp.val)
                node.append(temp.left)
                node.append(temp.right)
        return res[::-1]
# 递归求解
# return self.postorderTraversal(root.left) + self.postorderTraversal(root.right) + [root.val] if root is not None else []          

4.层次遍历(Level Order Traversal)－从上到下

https://leetcode.com/problems/binary-tree-level-order-traversal/

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

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        result, level = [], [root]
        while level:
            result.append([node.val for node in level])
            temp = []
            for node in level:
                temp.append(node.left)
                temp.append(node.right)
            level = [node for node in temp if node]
        return result

5.层次遍历(Level Order Traversal)－从下到上

https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

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

class Solution(object):
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        result, level = [], [root]
        while level:
            result.insert(0, [node.val for node in level])
            temp = []
            for node in level:
                temp.append(node.left)
                temp.append(node.right)
            level = [node for node in temp if node]
        return result

6.之字形遍历(Zigzag Level Order Traversal)

https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/

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

class Solution(object):
    def zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        result, level, direction = [], [root], 1
        while level:
            result.append([node.val for node in level][::direction])
            direction *= -1
            temp = []
            for node in level:
                temp.append(node.left)
                temp.append(node.right)
            level = [node for node in temp if node]
        return result