114. 二叉树展开为链表 #

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public void flatten(TreeNode root) {
        List<TreeNode> list = new ArrayList<TreeNode>();
        preOrderTraversal(root, list);
        for (int i = 1; i < list.size(); i++) {
            TreeNode prev = list.get(i - 1), current = list.get(i);
            prev.left = null;
            prev.right = current;
        }
    }

    private void preOrderTraversal(TreeNode node, List<TreeNode> list) {
        if (node == null) {
            return;
        }
        list.add(node);
        preOrderTraversal(node.left, list);
        preOrderTraversal(node.right, list);
    }
}