Invert a binary tree. 4 / \ 2 7 / \ / \1 3 6 9 to4 / \ 7 2 / \ / \9 6 3 1 **Trivia:**This problem was inspired by this original tweet by Max Howell:Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

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題目

翻轉一棵二叉樹

分析

Approach #1 (Recursive) [Accepted]

遞迴交換左右子樹即可

public TreeNode invertTree(TreeNode root) {
    if (root == null) {
        return null;
    }
    TreeNode right = invertTree(root.right);
    TreeNode left = invertTree(root.left);
    root.left = right;
    root.right = left;
    return root;
}
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Approach #2 (Iterative) [Accepted]

利用一個佇列即可，就像是層次遍歷，不過在裡面多加入一個交換的操作

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.left;
        current.left = current.right;
        current.right = temp;
        if (current.left != null) queue.add(current.left);
        if (current.right != null) queue.add(current.right);
    }
    return root;
}
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