101. 对称二叉树

深度优先搜索

class Solution {
    public boolean isSymmetric(TreeNode root) {

        return isSymmetric(root.left, root.right);
    }

    /**
     * 后序遍历
     * 从根节点的左右孩子开始递归
     */
    public boolean isSymmetric(TreeNode left, TreeNode right){

        /**
         * 除非全是null或全不是null,否则返回false
         */
        if (left == null || right == null){
            return left == right;
        }

        if (left.val != right.val){
            return false;
        }

        return isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(logn)
 */

迭代(队列)

class Solution {
    public boolean isSymmetric(TreeNode root) {

        /**
         * 迭代,使用队列的先进先出原则
         * 先将根节点的左右孩子入队,如果满足条件,再将左孩子的左孩子和右孩子的右孩子、左孩子的右孩子和右孩子的左孩子入队
         * 每次出队两个节点进行比较
         */
        Queue<TreeNode> queue = new LinkedList<>();

        queue.add(root.left);
        queue.add(root.right);

        while (!queue.isEmpty()){

            TreeNode left = queue.poll();
            TreeNode right = queue.poll();

            if (left == null && right == null){
                continue;
            }

            if (left == null || right == null){
                return false;
            }

            if (left.val != right.val){
                return false;
            }

            queue.add(left.left);
            queue.add(right.right);

            queue.add(left.right);
            queue.add(right.left);
        }

        return true;
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(n)
 */

迭代(栈)

class Solution {
    public boolean isSymmetric(TreeNode root) {

        /**
         * 在本题中,使用栈和队列是一模一样的
         */
        Stack<TreeNode> stack = new Stack<>();

        stack.push(root.left);
        stack.push(root.right);

        while (!stack.isEmpty()){

            TreeNode left = stack.pop();
            TreeNode right = stack.pop();

            if (left == null && right == null){
                continue;
            }

            if (left == null || right == null){
                return false;
            }

            if (left.val != right.val){
                return false;
            }

            stack.push(left.left);
            stack.push(right.right);

            stack.push(left.right);
            stack.push(right.left);
        }

        return true;
    }
}

https://leetcode-cn.com/problems/symmetric-tree/

posted @ 2021-10-27 18:36  振袖秋枫问红叶  阅读(27)  评论(0)    收藏  举报