使用递归的方法咯

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