FlatenBinaryTreeToLinkedList.java

/**
 * Given a binary tree, flatten it to a linked list in-place.
 *
 * For example,
 * Given
 * |         1
 * |        / \
 * |       2   5
 * |      / \   \
 * |     3   4   6
 * The flattened tree should look like:
 * |   1
 * |    \
 * |     2
 * |      \
 * |       3
 * |        \
 * |         4
 * |          \
 * |           5
 * |            \
 * |             6
 * |
 * Hints:
 * If you notice carefully in the flattened tree, each node's right child
 * points to the next node of a pre-order traversal.
 *
 * Tags: Tree, DFS
 */
class FlatenBinaryTreeToLinkedList {
    public static void main(String[] args) {

    }

    /**
     * addRecursive root's right subtree to left node's rightmost child
     * Then set that subtree as root's right subtree
     * And set root's left child to null
     * Move root to its right child and repeat
     */
    public void flatten(TreeNode root) {
        while (root != null) {
            if (root.left != null) { // check left child
                TreeNode n = root.left;
                while (n.right != null) n = n.right; // rightmost child of left
                n.right = root.right; // insert right subtree to its right
                root.right = root.left; // set left subtree as right subtree
                root.left = null; // set left to null
            }
            root = root.right; // move to right child
        }
    }

    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }
}