LeetCode 114. 二叉树展开为链表

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LeetCode 114. 二叉树展开为链表 ，难度中等。

DFS

解题思路：先用 DFS 遍历二叉树，将树的结果存放在 List 中，然后更新原先的二叉树。

/**
 * 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 preorderTraversal(List<TreeNode> traList, TreeNode root) {
        if (root == null) {
            return;
        }
        traList.add(root);
        preorderTraversal(traList, root.left);
        preorderTraversal(traList, root.right);
    }

    public void flatten(TreeNode root) {
        List<TreeNode> traList = new ArrayList<>();
        preorderTraversal(traList, root);
        TreeNode temp = root;
        for (int i = 1; i < traList.size(); ++i) {
            temp.right = traList.get(i);
            temp.left = null;
            temp = temp.right;
        }
    }
}