100. 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.

关于树结构自己没怎么看过，查了查遍历通常是：递归、stack

自己的解法

/**
 * Definition for a binary tree node.
 * 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) {
   return false;
  }
  if (p.val == q.val) {
   return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
  }
  return false;
    }
}