写的代码：

class Solution {
public:string cc(int num){string res="";while(num>0){int r=num % 2;res=static_cast<char>(48+r)+res;num/=2;}return res;}int minChanges(int n, int k) {int res=0;string n2=cc(n);string k2=cc(k);int n_size=n2.size();int k_size=k2.size();if(k_size>n_size){return -1;}else{for(int i=0;i<k_size;i++){if(n2[n_size-i-1]=='0' && k2[k_size-i-1]=='1')return -1;else if(n2[n_size-i-1]=='1' && k2[k_size-i-1]=='0')res++;}}for(int i=0;i<n_size-k_size;i++){if(n2[i]=='1'){res++;}}return res;}
};

数字转字符 static_cast<char> 或者用 to_string()也可以