LeetCode101.java

package problems;

import java.util.LinkedList;

/**
 * DFS
 */
public class LeetCode101 {
    public boolean isSymmetric(TreeNode root) {
        return dfs(root.left, root.right);
    }

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

/**
 * 队列
 */
class LeetCode101_1 {
    public boolean isSymmetric(TreeNode root) {
        if(root==null || (root.left==null && root.right==null)) {
            return true;
        }
        //用队列保存节点
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        //将根节点的左右孩子放到队列中
        queue.add(root.left);
        queue.add(root.right);
        while(queue.size()>0) {
            //从队列中取出两个节点，再比较这两个节点
            TreeNode left = queue.removeFirst();
            TreeNode right = queue.removeFirst();
            //如果两个节点都为空就继续循环，两者有一个为空就返回false
            if(left==null && right==null) {
                continue;
            }
            if(left==null || right==null) {
                return false;
            }
            if(left.val!=right.val) {
                return false;
            }
            //将左节点的左孩子， 右节点的右孩子放入队列
            queue.add(left.left);
            queue.add(right.right);
            //将左节点的右孩子，右节点的左孩子放入队列
            queue.add(left.right);
            queue.add(right.left);
        }

        return true;
    }
}