LeetCode Symmetric Tree

LeetCode解题之Symmetric Tree

原题

判断一棵树是否是镜面对称的。最好同时提供递归和迭代的解法。

注意点：

• 无

例子：

输入:

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

输出: True

输入:

    1
   / \   2   2
   \   \    3    3

输出: False

解题思路

看一棵二叉树是否对称，就要首先看根节点的左右节点A和B是否有相同的值，如果A和B的值相等，那么要继续判断A的左节点和B的右节点以及A的右节点和B的左节点是否对称，通过这样的方式来递归得到结果。用迭代方法来解决的话，就把要判断是否对称的点按序（注意需要判断哪些节点是否相等）放到两个栈中，不断出栈和压栈来判断。

AC源码

# 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):
    # Solve it recursively
    def isSymmetric(self, root):
        """ :type root: TreeNode :rtype: bool """
        if not root:
            return True
        return self._isSymmetric(root.left, root.right)

    def _isSymmetric(self, left, right):
        if not left and not right:
            return True
        if not left or not right:
            return False
        if left.val != right.val:
            return False
        return self._isSymmetric(left.left, right.right) and self._isSymmetric(left.right, right.left)

    # Solve it iteratively
    def isSymmetric_iterate(self, root):
        """ :type root: TreeNode :rtype: bool """
        if not root:
            return True
        stack1, stack2 = [], []
        stack1.append(root.left)
        stack2.append(root.right)
        while stack1 and stack2:
            size1 = len(stack1)
            size2 = len(stack2)
            if size1 != size2:
                return False
            for __ in range(size1):
                curr1, curr2 = stack1.pop(), stack2.pop()
                if not curr1 and not curr2:
                    continue
                if not curr1 or not curr2:
                    return False
                if curr1.val != curr2.val:
                    return False
                stack1.append(curr1.left)
                stack1.append(curr1.right)
                stack2.append(curr2.right)
                stack2.append(curr2.left)
        return not stack1 and not stack2

if __name__ == "__main__":
    None

欢迎查看我的Github (https://github.com/gavinfish/LeetCode-Python) 来获得相关源码。