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 binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {
        if(p==null && q==null) return true;
        if(p==null && q!=null || p!=null && q==null) return false;
        return isSameTree(p.left,q.left)&&(p.val==q.val)&&isSameTree(p.right,q.right);
    }
}