100. Same Tree

class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {
        return dfs(p, q);
    }
    
    private boolean dfs(TreeNode first, TreeNode second){
        // preorder
        if(first == null || second == null){
            return first==second;
        }
        if (first.val!=second.val) return false;
        
        if(dfs(first.left, second.left)){
            return dfs(first.right, second.right);
        }else{
            return false;
        }
    }
}