java 101.对称树（＃1 Recursive）.java

Posted 2021-05-08

public boolean isSymmetric(TreeNode root) {
    if(root==null)  return true;
    
    Stack<TreeNode> stack = new Stack<TreeNode>();
    TreeNode left, right;
    if(root.left!=null){
        if(root.right==null) return false;
        stack.push(root.left);
        stack.push(root.right);
    }
    else if(root.right!=null){
        return false;
    }
        
    while(!stack.empty()){
        if(stack.size()%2!=0)   return false;
        right = stack.pop();
        left = stack.pop();
        if(right.val!=left.val) return false;
        
        if(left.left!=null){
            if(right.right==null)   return false;
            stack.push(left.left);
            stack.push(right.right);
        }
        else if(right.right!=null){
            return false;
        }
            
        if(left.right!=null){
            if(right.left==null)   return false;
            stack.push(left.right);
            stack.push(right.left);
        }
        else if(right.left!=null){
            return false;
        }
    }
    
    return true;
}

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    
    def helper(self,left,right):
        if not left and not right:
            return True;
        elif not left or not right:
            return False;
        elif left.val != right.val:
            return False;
        else:
            return self.helper(left.left,right.right) and self.helper(left.right,right.left);
        
    def isSymmetric(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root:
            return True;
        
        return self.helper(root.left,root.right);
        
        
    
            

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) { return true; }
        Queue<TreeNode> l = new LinkedList<>();
        Queue<TreeNode> r = new LinkedList<>();
        l.offer(root.left);
        r.offer(root.right);
        while(!l.isEmpty() && !r.isEmpty()) {
            TreeNode a = l.poll(), b = r.poll();
            if (a == null && b == null) continue;
            if (a == null || b == null) return false;
            if (a.val != b.val) return false;
            l.offer(a.left);
            r.offer(b.right);
            l.offer(a.right);
            r.offer(b.left);
        }
        return true;
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean helper(TreeNode root1, TreeNode root2) {
        if (root1 == null && root2 == null) return true;
        if (root1 == null || root2 == null || root1.val != root2.val) return false;
        return helper(root1.left, root2.right) && helper(root1.right, root2.left);
    }
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return helper(root.left, root.right);
    }
}