试水

public class Solution {
    public String decode (String str, int d) {
        String order="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
        int len=str.length();
        char []ans=str.toCharArray();
        int index=0;
        for(int i=0;i<len;i++){
            if(ans[i]>='a'&&ans[i]<='z')
                index=36+(ans[i]-'a');
            else if(ans[i]>='A'&&ans[i]<='Z')
                index=10+(ans[i]-'A');
            else
                index=ans[i]-'0';
            ans[i]=order.charAt((62+index-d)%62);
            }
        return new String(ans);
    }
}