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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

/*
DFS, recursion 
time:O(n), space:O(1)
*/

public class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {
        if (p == null || q == null) {
            return p == q;
        }
        if (p.val != q.val) {
            return false;
        }
        return isSameTree(p.left, q.left) && 
               isSameTree(p.right, q.right);
    }
}