leetcode -- Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / 
  2   2
 /  / 
3  4 4  3

 But the following is not:

    1
   / 
  2   2
      
   3    3

 Note:

Bonus points if you could solve it both recursively and iteratively.

递归解法

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(root == null){
            return true;
        }
        
        //if(root.left != null && root.right != null){
            return cmp(root.left, root.right);
        //}
        
        //return false;
    }
    
    public boolean cmp(TreeNode l, TreeNode r){
        boolean f1 = false, f2 = false;
        if(l == null && r == null){
            return true;
        }
        
        if((l != null && r == null) ||
            (l == null && r != null)){
                return false;
            }

        if(l.val != r.val){
            return false;
        }
        //if(l.left != null && r.right != null){
            f1 = cmp(l.left, r.right);
        //}
        //if(l.right != null && r.left != null){
            f2 = cmp(l.right, r.left);
        //}
        //if(l != null && r != null && f1 && f2){
        //  return false;
        //}
        if(f1 && f2){
            return true;
        }
        return false;
        
    }
}

 迭代版本(层序遍历)

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(root == null){
            return true;
        }
        LinkedList<TreeNode> l = new LinkedList<TreeNode>(),
            r = new LinkedList<TreeNode>();
            
        l.add(root.left);
        r.add(root.right);
        while(!l.isEmpty() && !r.isEmpty()){
            TreeNode t1 = l.poll(), t2 = r.poll();
            if((t1 == null && t2 != null) || (t1 != null && t2 == null)){
                return false;            
            }
            if(t1 != null){
                if(t1.val != t2.val){
                    return false;
                }
                l.add(t1.left);
                l.add(t1.right);
                r.add(t2.right);
                r.add(t2.left);
            }
        }
        return true;
    }
}