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.

Analyse: Compare all nodes in the same place.

1. Recursion

    Runtime: 0ms.

/**
 * 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 && !q) return true;
        if(!p || !q) return false; //the first line eliminated the situation s.t. both trees are null
        return p->val == q->val &&
               isSameTree(p->left, q->left) &&
               isSameTree(p->right, q->right);
    }
};

2. Iteration with stack used

    Runtime: 0ms.

/**
 * 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) {
        stack<TreeNode* > stk;
        stk.push(p);
        stk.push(q);
        
        while(!stk.empty()){
            p = stk.top();
            stk.pop();
            q = stk.top();
            stk.pop();
            
            if(!p && !q) continue;
            if(!p || !q) return false;
            if(p->val != q->val) return false;
            
            stk.push(p->left);
            stk.push(q->left);
            stk.push(p->right);
            stk.push(q->right);
        }
        return true;
    }
};

3. Iteration with queue used

    Runtime: 4ms.

/**
 * 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) {
        queue<TreeNode* > qu;
        qu.push(p);
        qu.push(q);
        
        while(!qu.empty()){
            p = qu.front();
            qu.pop();
            q = qu.front();
            qu.pop();
            
            if(!p && !q) continue;
            if(!p || !q) return false;
            if(p->val != q->val) return false;
            
            qu.push(p->left);
            qu.push(q->left);
            qu.push(p->right);
            qu.push(q->right);
        }
        return true;
    }
};