c# leetcode1079. 活字印刷(回溯算法)

 

你有一套活字字模 tiles，其中每个字模上都刻有一个字母 tiles[i]。返回你可以印出的非空字母序列的数目。

 

示例 1：

输入："AAB"
输出：8
解释：可能的序列为 "A", "B", "AA", "AB", "BA", "AAB", "ABA", "BAA"。
示例 2：

输入："AAABBC"
输出：188
 

提示：

1 <= tiles.length <= 7
tiles 由大写英文字母组成 

 

public class Solution {
        public int NumTilePossibilities(string tiles)
        {
            var set = new HashSet();
            var list = tiles.ToList();

            // brute force all lengths
            // length options: 1, 2, 3, 4, 5, 6, 7
            for (int i = 1; i <= tiles.Length; i++)
            {
                NumTilePossibilitiesHelper(new StringBuilder(), set, list, i);
            }

            return set.Count;
        }

     private static void NumTilePossibilitiesHelper(
            StringBuilder sequence, 
            HashSet set, 
            List tiles, 
            int length)
        {
            // base case 
            if (sequence.Length == length)
            {
                set.Add(sequence.ToString());
                return;
            }

            // please consider the next char for all possible chars
            int backtrackingLength = sequence.Length;

            for (int i = 0; i < tiles.Count; i++)
            {
                var current = tiles[i];

                sequence.Append(current);
                // remove current char from the list
                tiles.RemoveAt(i);

                NumTilePossibilitiesHelper(sequence, set, tiles, length);

                // insert current char to the list
                // List.Insert()
                tiles.Insert(i, current);

                // StringBuilder.Remove(start, length)
                sequence.Remove(backtrackingLength, sequence.Length - backtrackingLength);
            }
        }
}