572. Subtree of Another Tree

题目：

Given two non-empty binary trees s and t, check whether tree t has exactly the same structure and node values with a subtree of s. A subtree of s is a tree consists of a node in s and all of this node's descendants. The tree s could also be considered as a subtree of itself.

Example 1:
Given tree s:

     3    / \   4   5  / \ 1   2

Given tree t:

   4   / \ 1   2

Return true, because t has the same structure and node values with a subtree of s.

Example 2:
Given tree s:

     3    / \   4   5  / \ 1   2    /   0

Given tree t:

   4  / \ 1   2

Return false.

思路：

本题采用递归的思路，遍历s的每个结点，判断以当前结点为根结点的树是否与待比较的树一样，一样返回true,否则返回false

代码：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool isSubtree(TreeNode* s, TreeNode* t) {        if (!s) return false;        if (isSame(s, t)) return true;        return isSubtree(s->left, t) || isSubtree(s->right, t);    }    bool isSame(TreeNode* s, TreeNode* t) {        if (!s && !t) return true;        if (!s || !t) return false;        if (s->val != t->val) return false;        return isSame(s->left, t->left) && isSame(s->right, t->right);    }};