【Leetcode】【Easy】Same Tree

Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

递归的解法：

用树递归的思想，将两个树以结点作为比较单位，只关注对当前位置结点的操作（是否都存在此结点，存在时值是否相同）。

注意：

1、只关注单个结点，结点的孩子交给递归去做，这样可以最简化逻辑；

2、结点是否存在、结点值是否相等、结点是否有孩子，是三个不同的概念（“结点是否有孩子”的概念本题中未体现）；

 /**
  * Definition for binary tree
  * 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 && !q)
             return true;

         if ((p && !q) || (!p && q) || (p->val != q->val))
             return false;

         return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
     }
 };

迭代的解法：

引用先序/中序/后序/按层等遍历二叉树的思想，用堆栈（数组）存放遍历到的结点，进栈出栈的同时进行比较。

中序遍历（其他方法类似）：

 /**
  * Definition for binary tree
  * 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) {
         vector<TreeNode *> stackP;
         vector<TreeNode *> stackQ;
         TreeNode *currentP = p;
         TreeNode *currentQ = q;

         while (currentP || stackP.size()) {
             if (!currentP && !currentQ) {
                 currentP = stackP.back();
                 currentQ = stackQ.back();
                 stackP.pop_back();
                 stackQ.pop_back();
                 currentP = currentP->right;
                 currentQ = currentQ->right;
                 continue;
             }

             if ((!currentP && currentQ) || (currentP && !currentQ) ||\
                 (currentP->val != currentQ->val))
                 return false;

             if (currentP->val == currentQ->val) {
                 stackP.push_back(currentP);
                 stackQ.push_back(currentQ);
                 currentP = currentP->left;
                 currentQ = currentQ->left;
             }
         }

         if (!currentQ) {
             return true;
         } else {
             return false;
         }

     }
 };

附录：

C++中vector、stack、queue用法和区别

递归/非递归遍历二叉树