572. Subtree of Another Tree（easy）

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： 本质上还是树的遍历，考虑细节，递归中有递归。 

/*
可以结合Leetcode 100 same tree 那道题来理解，这里要求是求一个树的子树是否包含另一个树，所以要遍历一遍第一个树（这里用先序遍历）。
所以递归函数中的函数也是递归函数，很有意思。

*/
/**
 * 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 sameTree(TreeNode * s, TreeNode * t){
        if (s == NULL && t == NULL){
            return true;
        }
        if (s == NULL || t == NULL){
            return false;
        }
        return s -> val == t -> val && sameTree(s -> left, t -> left) && sameTree(s -> right, t -> right);
    }
    bool isSubtree(TreeNode* s, TreeNode* t) {
        if (s == NULL){      // 若是第一个树到了末尾还没有找到则就是返回false
            return NULL;
        }
        if (sameTree(s , t)){
            return true;
        }
        return isSubtree(s -> left, t) || isSubtree(s -> right, t);
    }
};

 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

 两道题有共同点。这一道题其实就是上一道题中的一部分，就是判断是否是相等的两个树、

 

/**
 * 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 isSameTree(TreeNode* p, TreeNode* q) {
        if (p == NULL && q == NULL){
            return true;
        }
        if (p == NULL || q == NULL){
            return false;
        }
        return p -> val == q -> val && isSameTree(p -> left, q -> left) && isSameTree(p -> right, q -> right);
    }
};