Subtree

Question

• lintcode: (245) Subtree

You have two every large binary trees: T1,
with millions of nodes, and T2, with hundreds of nodes.
Create an algorithm to decide if T2 is a subtree of T1.

Example
T2 is a subtree of T1 in the following case:
       1                3
      / \              /
T1 = 2   3      T2 =  4
        /
       4
T2 isn't a subtree of T1 in the following case:
       1               3
      / \               \
T1 = 2   3       T2 =    4
        /
       4
Note
A tree T2 is a subtree of T1 if there exists a node n in T1 such that
the subtree of n is identical to T2.
That is, if you cut off the tree at node n,
the two trees would be identical.

题解

判断 T2是否是 T1的子树，首先应该在 T1中找到 T2的根节点，找到根节点后两棵子树必须完全相同。所以整个思路分为两步走：找根节点，判断两棵树是否全等。咋看起来极其简单，但实际实现中还是比较精妙的，尤其是递归的先后顺序及条件与条件或的处理。

• reference

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        this.val = val
        this.left, this.right = None, None
"""
class Solution:
    # @param T1, T2: The roots of binary tree.
    # @return: True if T2 is a subtree of T1, or false.
    def get(self, root, rt):
        if root is None:
            rt.append("#")
            return

        rt.append(str(root.val))
        self.get(root.left, rt)
        self.get(root.right, rt)

    def isSubtree(self, T1, T2):
        # write your code here
        rt = []
        self.get(T1, rt)
        t1 = ','.join(rt)

        rt = []
        self.get(T2, rt)
        t2 = ','.join(rt)

        return t1.find(t2) != -1