LeetCode: isSameTree1 解题报告

isSameTree1

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.

SOLUTION 1 & SOLUTION 2:

以下是递归及非递归解法:

1. 递归解法就是判断当前节点是不是相同，及左右子树是不是相同树

2. 非递归解法使用了先根遍历。这个算法比较简单一点

 /**
  * Definition for binary tree
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     // solution 1:
     public boolean isSameTree1(TreeNode p, TreeNode q) {
         if (p == null && q == null) {
             return true;
         }

         if (p == null || q == null) {
             return false;
         }

         return p.val == q.val &&
            isSameTree(p.left, q.left) &&
            isSameTree(p.right, q.right);
     }

     // Solution 2:
     public boolean isSameTree(TreeNode p, TreeNode q) {
         if (p == null && q == null) {
             return true;
         }

         if (p == null || q == null) {
             return false;
         }

         Stack<TreeNode> s1 = new Stack<TreeNode>();
         Stack<TreeNode> s2 = new Stack<TreeNode>();

         s1.push(p);
         s2.push(q);

         while (!s1.isEmpty() && !s2.isEmpty()) {
             TreeNode cur1 = s1.pop();
             TreeNode cur2 = s2.pop();

             // 弹出的节点的值必须相等
             if (cur1.val != cur2.val) {
                 return false;
             }

             // tree1的right节点，tree2的right节点，可以同时不为空，也可以同时为空，否则返回false.
             if (cur1.left != null && cur2.left != null) {
                 s1.push(cur1.left);
                 s2.push(cur2.left);
             } else if (!(cur1.left == null && cur2.left == null)) {
                 return false;
             }

             // tree1的左节点，tree2的left节点，可以同时不为空，也可以同时为空，否则返回false.
             if (cur1.right != null && cur2.right != null) {
                 s1.push(cur1.right);
                 s2.push(cur2.right);
             } else if (!(cur1.right == null && cur2.right == null)) {
                 return false;
             }
         }

         return true;
     }
 }

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/tree/IsSameTree1.java