LeetCode 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.

分析：二叉树的问题，可以选择递归算法，也可以选择遍历二叉树，对应结点相等的方式来判断。

解法一：非递归算法，需要考虑多种情况。

/**
 * 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) {
        LinkedList<TreeNode> stack1 = new LinkedList<TreeNode>();
        LinkedList<TreeNode> stack2 = new LinkedList<TreeNode>();
        if(p == null && q == null){
            return true;
        }else if(p != null && q != null){
            stack1.add(p);
            stack2.add(q);
            while(stack1.size() != 0 && stack2.size() != 0){
                TreeNode node1 = stack1.removeFirst();
                TreeNode node2 = stack2.removeFirst();
                if(node1 == null && node2 == null){
                    continue;
                }else if(node1 != null && node2 != null){
                    if(node1.val != node2.val){
                        return false;
                    }
                    stack1.add(node1.left);
                    stack2.add(node2.left);
                    stack1.add(node1.right);
                    stack2.add(node2.right);
                }else{
                    return false;
                }
            }
            return true;
        }else{
            return false;
        }
        
    }
}

代码有优化的空间，将上述的条件中pq同为null或者同不为null的情况合并：

/**
 * 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) {
        LinkedList<TreeNode> stack1 = new LinkedList<TreeNode>();
        LinkedList<TreeNode> stack2 = new LinkedList<TreeNode>();
        if((p == null && q == null) || (p != null && q != null)){
            stack1.add(p);
            stack2.add(q);
            while(stack1.size() != 0 && stack2.size() != 0){
                TreeNode node1 = stack1.removeFirst();
                TreeNode node2 = stack2.removeFirst();
                if(node1 == null && node2 == null){
                    continue;
                }else if(node1 != null && node2 != null){
                    if(node1.val != node2.val){
                        return false;
                    }
                    stack1.add(node1.left);
                    stack2.add(node2.left);
                    stack1.add(node1.right);
                    stack2.add(node2.right);
                }else{
                    return false;
                }
            }
            return true;
        }else{
            return false;
        }   
        
    }
}

同样，本题的递归算法代码如下：

/**
 * 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) {
        // 非递归算法
        // LinkedList<TreeNode> stack1 = new LinkedList<TreeNode>();
        // LinkedList<TreeNode> stack2 = new LinkedList<TreeNode>();
        // if((p == null && q == null) || (p != null && q != null)){
        //     stack1.add(p);
        //     stack2.add(q);
        //     while(stack1.size() != 0 && stack2.size() != 0){
        //         TreeNode node1 = stack1.removeFirst();
        //         TreeNode node2 = stack2.removeFirst();
        //         if(node1 == null && node2 == null){
        //             continue;
        //         }else if(node1 != null && node2 != null){
        //             if(node1.val != node2.val){
        //                 return false;
        //             }
        //             stack1.add(node1.left);
        //             stack2.add(node2.left);
        //             stack1.add(node1.right);
        //             stack2.add(node2.right);
        //         }else{
        //             return false;
        //         }
        //     }
        //     return true;
        // }else{
        //     return false;
        // }
        //递归算法
        if(p == null && q == null) return true;
        else if(p == null || q == null) return false;
        else{
            if(p.val != q.val) return false;
        }
        boolean left = isSameTree(p.left, q.left);
        boolean right = isSameTree(p.right, q.right);
        return left && right;
    }
}