[LeetCode 101] Symmetric Tree

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Description

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Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

Example 1

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• Input: root = [1,2,2,3,4,4,3]
• Output: true

Example 2

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• Input: root = [1,2,2,null,3,null,3]
• Output: false

Constraints

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• The number of nodes in the tree is in the range [1, 1000].
• -100 <= Node.val <= 100

Code

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내 제출

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        return dfs(root.left, root.right);
    }

    private boolean dfs(TreeNode root1, TreeNode root2) {
        if (root1 == root2) {
            return true;
        }
        if (root1 == null || root2 == null || root1.val != root2.val) {
            return false;
        }
        return dfs(root1.left, root2.right) && dfs(root1.right, root2.left);
    }
}

Runtime	Memory
0 ms	41.8 MB

다른 풀이

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {

        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);

        while (!q.isEmpty()) {

            int size = q.size();
            List<Integer> ans = new ArrayList<>();

            for (int i = 0; i < size; i++) {

                TreeNode node = q.poll();

                // store left child value
                if (node.left != null) {
                    ans.add(node.left.val);
                    q.offer(node.left);
                } else {
                    ans.add(null);
                }

                // store right child value
                if (node.right != null) {
                    ans.add(node.right.val);
                    q.offer(node.right);
                } else {
                    ans.add(null);
                }
            }

            // palindrome check
            int l = 0;
            int r = ans.size() - 1;

            while (l < r) {

                Integer a = ans.get(l);
                Integer b = ans.get(r);

                if (a == null && b == null) {
                    l++;
                    r--;
                    continue;
                }

                if (a == null || b == null || !a.equals(b)) {
                    return false;
                }

                l++;
                r--;
            }
        }

        return true;
    }
}

Reference

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• https://github.com/doocs/leetcode/blob/main/solution/0100-0199/0101.Symmetric%20Tree/Solution.java