leetcode691 Stickers to Spell Word

思路：
状压dp。
实现：
 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 class Solution 
 4 {
 5 public:
 6     int minStickers(vector<string>& stickers, string target) 
 7     {
 8         vector<vector<int>> v(stickers.size(), vector<int>(26, 0));
 9         for (int i = 0; i < stickers.size(); i++)
10         {
11             for (int j = 0; j < stickers[i].size(); j++)
12             {
13                 v[i][stickers[i][j] - 'a']++;
14             }
15         }
16         vector<int> dp(1 << target.size(), -1);
17         int ret = dfs(0, v, target, dp);
18         if (ret < 0x3f3f3f3f) return ret;
19         return -1;
20     }
21     int dfs(int now, vector<vector<int>>& v, string target, vector<int>& dp)
22     {
23         if (now == (1 << target.size()) - 1) return 0;
24         if (dp[now] != -1) return dp[now];
25         int minn = 0x3f3f3f3f;
26         
27         for (int i = 0; i < v.size(); i++)
28         {
29             vector<int> tmp = v[i];
30             int s = now;
31             for (int j = 0; j < target.size(); j++)
32             {
33                 if (!(s & (1 << j)) && tmp[target[j] - 'a'])
34                 {
35                     s |= (1 << j); tmp[target[j] - 'a']--;
36                 }
37             }
38             if (s != now) { minn = min(minn, dfs(s, v, target, dp) + 1); }
39         }
40         return dp[now] = minn;
41     }
42 };