[JAVA] SWEA Professional 구간 합

문제 링크

SW Expert Academy

import java.util.*;
import java.io.*;

public class Solution {
	
	static long[] memo; 
	static int[] sum; // 1 ~ i 까지의 합

	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int T = Integer.parseInt(br.readLine());
		
		sum = new int[10];
		sum[1] = 1;
		for(int i=2;i<10;i++) sum[i] = i + sum[i-1];
		
		memo = new long[16];
		memo[1] = sum[9];
		for(int i=2; i<=15; i++) memo[i] =  memo[1] * (long)Math.pow(10, i-1) + memo[i-1]*10;
		
		for(int t=1;t<=T;t++) {
			StringTokenizer st = new StringTokenizer(br.readLine());
			long a = Long.parseLong(st.nextToken());
			long b = Long.parseLong(st.nextToken());
				
			System.out.println("#"+t+" "+(solve(b)-solve(a-1)));
		}
	}
	
	public static long solve(long num) {
        if(num<0) return 0;
		if(num<10) return sum[(int)num];
		
		String str = String.valueOf(num);
		int z = str.length()-1;
		int x = str.charAt(0) - '0';
		long y = Long.parseLong(str.substring(1));
		
		return sum[x-1]* (long) Math.pow(10, z) + x*memo[z] + x*(y+1) + solve(y);
	}

}