leetcode 100
    https://leetcode-cn.com/problems/same-tree/

    1. //DFS
    2. /**
    3.  * Definition for a binary tree node.
    4.  * public class TreeNode {
    5.  *     int val;
    6.  *     TreeNode left;
    7.  *     TreeNode right;
    8.  *     TreeNode() {}
    9.  *     TreeNode(int val) { this.val = val; }
    10.  *     TreeNode(int val, TreeNode left, TreeNode right) {
    11.  *         this.val = val;
    12.  *         this.left = left;
    13.  *         this.right = right;
    14.  *     }
    15.  * }
    16.  */
    17. class Solution {
    18.     public boolean isSameTree(TreeNode p, TreeNode q) {
    19.         return compare(p, q);
    20.     }
    21.     public boolean compare(TreeNode p, TreeNode q){
    22.         if(p == null && q != null){
    23.             return false;
    24.         }
    25.         else if(p != null && q == null){
    26.             return false;
    27.         }
    28.         else if(p == null && q == null){
    29.             return true;
    30.         }
    31.         else if(p.val != q.val){
    32.             return false;
    33.         }
    35.         boolean left = compare(p.left, q.left);
    36.         boolean right = compare(p.right, q.right);
    37.         return left & right; 
    38.     }
    39. }