Invert a binary tree.

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to

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算法分析

可以使用兩種方法實現：遞歸和迭代。
遞歸是從最下面的節點開始反轉，而迭代是從上面的節點開始。

方法一：遞歸調用

Java代碼

/**
 * 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 null;
        
        TreeNode left = invertTree(root.left);
        TreeNode right = invertTree(root.right);
        
        root.left = right;
        root.right = left;
        return root;
    }
}

方法二：迭代

java代碼

/**
 * 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 null;
        
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        while(!queue.isEmpty()) {
            TreeNode current = queue.poll();
            TreeNode temp = current.right;
            current.right = current.left;
            current.left = temp;
            
            if (current.left != null) queue.add(current.left);
            if (current.right != null) queue.add(current.right);
        }
        return root;
    }
}

參考

LeetCode