leetcode: 100. Same Tree

# Given two binary trees, write a function to check if they are the same or not.
#
# Two binary trees are considered the same if they are structurally identical 
# and the nodes have the same value.
#
#
# Example 1:
#
# Input:
#       1         1
#      / \       / \
#     2   3     2   3
#
#   [1,2,3],   [1,2,3]
#
# Output: true
# Example 2:
#
# Input:
#       1         1
#      /           \
#     2             2
#
#   [1,2],     [1,null,2]
#
# Output: false
# Example 3:
#
# Input:
#       1         1
#      / \       / \
#     2   1     1   2
#
#   [1,2,1],   [1,1,2]
#
# Output: false

对两棵树的每个相同位置都进行比较。一旦某结点不一致，即返False。

判断顺序：
1. not node1 and not node2:    continue
2. None in (node1, node2) 或 node1.val != node2.val:    返False
3. node1.val == node2.val:    继续判断左右子树

# Time:  O(n)
# Space: O(h)
class Solution():
    def isSameTree(self, p, q):
        stack = [(p, q)]
        while stack:
            node1, node2 = stack.pop()
            if not node1 and not node2:
                continue
            if None in (node1, node2) or node1.val != node2.val:
                return False
            else:
                stack.append((node1.left, node2.left))
                stack.append((node1.right, node2.right))
        return True

# Time:  O(n)
# Space: O(h)
class Solution():
    def isSameTree(self, p, q):
        if not p and not q:
            return True
        if None in (p, q) or p.val != q.val:
            return False
        else:
            return self.isSameTree(p.left, q.left) and self.isSameTree(p.right, q.right)