[每日 LeetCode] 572. Subtree of Another Tree

原文链接 [每日 LeetCode] 572. Subtree of Another Tree

Description:

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：本题要求判断一个树是不是另一棵树的子树，要求不能是中间的树，只能是从叶子结点开始。可以使用[每日 LeetCode] 100. Same Tree 代码，对原始树遍历，如果是同一棵树就返回 true，否则返回 false。

C++ 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 (isSameTree(s,t))
            return true;
        return isSubtree(s->left,t) || isSubtree(s->right,t);
    }
    
    bool isSameTree(TreeNode* p, TreeNode* q) {
        if (p == NULL || q == NULL) 
            return (p == q);
    return (p->val == q->val && isSameTree(p->left, q->left) && isSameTree(p->right, q->right));
    }
};

运行时间：32ms

运行内存：21.2M