LeetCode 226 (Invert Binary Tree

LeetCode:226 invert-binary-tree
先序遍历：

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null)
            return null;
        
        if(root.left != null)
            invertTree(root.left);
        if(root.right != null) 
            invertTree(root.right);
        
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        
        return root;
    }
}

中序遍历：

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null)
            return null;
        if(root.left != null)
            invertTree(root.left);
       
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        
        0if(root.right != null) 
            invertTree(root.right);
        
        return root;
    }
}

后序遍历：

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null)
            return null;
        if(root.left != null)
            invertTree(root.left);
            if(root.right != null) 
            invertTree(root.right);
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;

        return root;
    }
}

层序遍历：

class Solution {
   public TreeNode invertTree(TreeNode root) {
        Queue<TreeNode> list = new LinkedList<>();
        
        if(root == null)
            return null;
        
        list.add(root);
        
        while(!list.isEmpty()) {
            TreeNode node = list.poll();
            TreeNode temp = node.left;
            node.left = node.right;
            node.right = temp;
            if(node.left != null)
                list.add(node.left);
            if(node.right != null)
                list.add(node.right);
        }
        
        return root;
    }
}