100. Same Tree

https://leetcode.com/problems/same-tree/description/
判断两棵二叉树是否相同（结构一样，相同位置结点一样）
思路：按相同顺序遍历，用一个全局flag，初始为true。每当两棵树的结点不相等，或者一棵树有结点而另一棵树为空时，flag=false

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

class Solution:
    def isSameTree(self, p, q):
        """
        :type p: TreeNode
        :type q: TreeNode
        :rtype: bool
        """
        self.flag = True  #全局变量
        self.travel(p, q)
        return self.flag

    def travel(self, p, q):
        #若p，q都非空，可以前序，中序，后序遍历，只要两棵树次序一致都可
        if p and q: 
            if p.val != q.val:
                self.flag = False
            self.travel(p.left, q.left)            
            self.travel(p.right, q.right)
        #若p，q有一个空，另一个非空
        elif not p and q:
            self.flag = False
        elif p and not q:
            self.flag = False
        #p，q都为空，返回。由于函数没有返回值，可以省略不写
        else:
            return