leetcode691 Stickers to Spell Word

思路：

状压dp。
实现：

#include <bits/stdc++.h>
using namespace std;
class Solution 
{
public:
    int minStickers(vector<string>& stickers, string target) 
    {
        vector<vector<int>> v(stickers.size(), vector<int>(26, 0));
        for (int i = 0; i < stickers.size(); i++)
        {
            for (int j = 0; j < stickers[i].size(); j++)
            {
                v[i][stickers[i][j] - 'a']++;
            }
        }
        vector<int> dp(1 << target.size(), -1);
        int ret = dfs(0, v, target, dp);
        if (ret < 0x3f3f3f3f) return ret;
        return -1;
    }
    int dfs(int now, vector<vector<int>>& v, string target, vector<int>& dp)
    {
        if (now == (1 << target.size()) - 1) return 0;
        if (dp[now] != -1) return dp[now];
        int minn = 0x3f3f3f3f;
        
        for (int i = 0; i < v.size(); i++)
        {
            vector<int> tmp = v[i];
            int s = now;
            for (int j = 0; j < target.size(); j++)
            {
                if (!(s & (1 << j)) && tmp[target[j] - 'a'])
                {
                    s |= (1 << j); tmp[target[j] - 'a']--;
                }
            }
            if (s != now) { minn = min(minn, dfs(s, v, target, dp) + 1); }
        }
        return dp[now] = minn;
    }
};