Invert Binary Tree 解答

Quetion

Invert a binary tree.

     4
   /   \
  2     7
 / \   / \
1   3 6   9

to

     4
   /   \
  7     2
 / \   / \
9   6 3   1

Solution 1 -- Recursion

Easy to think.

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public TreeNode invertTree(TreeNode root) {
         if (root == null)
             return root;
         TreeNode leftNode = null, rightNode = null;
         if (root.left != null)
             leftNode = invertTree(root.left);
         if (root.right != null)
             rightNode = invertTree(root.right);
         root.left = rightNode;
         root.right = leftNode;
         return root;
     }
 }

Solution 2 -- Iteration

BFS

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public TreeNode invertTree(TreeNode root) {
         if (root == null)
             return root;
         List<TreeNode> current = new ArrayList<TreeNode>();
         List<TreeNode> next;
         current.add(root);
         while (current.size() > 0) {
             next = new ArrayList<TreeNode>();
             for (TreeNode tmpNode : current) {
                 // inverse tmpNode
                 TreeNode left = tmpNode.left;
                 TreeNode right = tmpNode.right;
                 tmpNode.right = left;
                 tmpNode.left = right;
                 if (right != null)
                     next.add(right);
                 if (left != null)
                     next.add(left);
             }
             current = next;
         }
         return root;
     }
 }