官方题解https://leetcode-cn.com/problems/UlBDOe/solution/qiu-xie-shou-cang-ji-by-leetcode-solution/
class Solution { public: int minimumOperations(string leaves) { int n=leaves.size(); int f[n][3]; f[0][0]=(leaves[0]=='y'), f[0][1]=f[0][2]=f[1][2]=INT_MAX; for(int i=1;i<n;++i){ f[i][0]=f[i-1][0]+(leaves[i]=='y'); f[i][1]=min(f[i-1][0], f[i-1][1])+(leaves[i]=='r'); if(i>1) f[i][2]=min(f[i-1][1], f[i-1][2])+(leaves[i]=='y'); } return f[n-1][2]; } };
