100.[Tree][Easy] Same Tree

Problem

https://leetcode.com/problems/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

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
// recursion
// 注意判断p和q是否都为空或其中一个为空
// Complexity:
// Space: O(h) h为树高度，best h = logn 树平衡， worst h = n 树不平衡
// Time: O(n) 
class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {
        if(p == null || q == null) {
            return p == q;
        }
        
        if (p.val == q.val) {
            return(isSameTree(p.left, q.left) && isSameTree(p.right, q.right));
        }
        
        return false; 
    }
}

// Iteration 用stack，分别放入两个stack，在pop时比较。