LeetCode 1079. 活字印刷（全排列回溯）

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

class Solution {
public:
    int numTilePossibilities(string tiles) {
        int kinds = 0;
        sort(tiles.begin(), tiles.end());
        for(int sublen = 1; sublen <= tiles.size(); ++sublen)
        {
        	bt(tiles,kinds,0,tiles.size()-1,sublen,0);
        }       
        return kinds;
    }
    void bt(string tiles, int &kinds, int left, int right, int sublen, int curlen)
    {
    	if(curlen == sublen)
    		++kinds;
    	else
    	{
    		for(int i = left; i <= right; ++i)
	    	{
	    		if(i > left && tiles[i] == tiles[left])
	    			continue;
	    		swap(tiles[left],tiles[i]);
	    		bt(tiles,kinds,left+1,right,sublen,curlen+1);
	    	}
    	}  	
    }
};