程序员面试金典-面试题 04.09. 二叉搜索树序列

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> BSTSequences(TreeNode root) {
        res = new LinkedList<>();
        if(root == null){
            res.add(new LinkedList<>());
            return res;
        }
        LinkedList<TreeNode> list = new LinkedList<>();
        LinkedList<Integer> path = new LinkedList<>();
        path.add(root.val);
        getSequences(root, list, path);
        return res;
    }
    private void getSequences(TreeNode root, LinkedList<TreeNode> list, LinkedList<Integer> path){
        if(root == null){
            return;
        }
        if(root.left != null)
            list.add(root.left);
        if(root.right != null)
            list.add(root.right);
        if(list.isEmpty()){
            res.add(new LinkedList<>(path));
            return;
        }
        int len = list.size();
        for(int i = 0; i < len; ++i){
            TreeNode cur = list.get(i);
            list.remove(i);
            path.add(cur.val);
            getSequences(cur, new LinkedList<>(list), path);
            path.removeLast();
            list.add(i, cur);
        }
    }
    private List<List<Integer>> res;
}