101. 对称二叉树

递归

/**
 * 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) {
        //递归版本
        return isMirror(root,root);
    }
    public boolean isMirror(TreeNode r1,TreeNode r2) {
        if(r1 == null && r2 == null ) return true;
        if(r1 == null || r2 == null ) return false;
        return r1.val == r2.val && isMirror(r1.left,r2.right) && isMirror(r1.right,r2.left);
    }
}

迭代

/**
 * 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> que = new LinkedList<>();
        que.add(root);
        que.add(root);
        while(! que.isEmpty() ) {
            TreeNode t1 = que.poll();
            TreeNode t2 = que.poll();
            if(t1 == null && t2 == null ) continue;
            if(t1 == null || t2 == null ) return false;
            if(t1.val != t2.val ) return false;
            que.add(t1.left);
            que.add(t2.right);
            que.add(t1.right);
            que.add(t2.left);
        }
        return true;
    }
}