[C/C++] 프로젝트 오일러 #13 BigInt 수의 합 구하기

프로젝트 오일러( Project Euler )의 13번 문제 Large Sum 은 다음과 같은 내용을 다룹니다:
• 문제에서는 50자리로 이루어진 큰 수(Big integer) 100개가 주어집니다.
• 이 100개의 수를 모두 더하여 얻은 합(누적값)을 구합니다.
• 그렇게 구한 합의 가장 앞에서부터 10자리만을 추출하여 결과로 제시해야 합니다.

즉, 핵심 요구사항은 “아래에 주어진 100개의 50자리 수를 모두 합한 뒤, 그 합의 처음 10자리를 구해라” 입니다.

 

사실 이번 문제는 꼼수로 푸는 것이 맞을 듯 합니다.

BigInt 라이브러리를 이용해서 풀어도 되겠지만, 그렇게 하면 시간이 많이 걸리죠.

 

앞자리부터 10자리 숫자만 구하면 되는 것이니까요.  그렇게 하면 int형으로는 위험할지 몰라도 int64형으로는 충분한 자릿수가 됩니다.  32비트 자료형은 10의 9승정도를 표현할 수 있으니까요.  64비트는 10의 19승까지 표현할 수 있죠.

 

large sum

 

제가 작성한 소스입니다.

//------------------------------------------------
//    Project Euler #13 - Large Sum
//        - by Aubrey Choi
//        - created at 2014-12-29
//------------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <time.h>
#include <math.h>
#include "isprime.h"

void solve1()
{
    char *s = "37107287533902102798797998220837590246510135740250"
        "46376937677490009712648124896970078050417018260538"
        "74324986199524741059474233309513058123726617309629"
        "91942213363574161572522430563301811072406154908250"
        "23067588207539346171171980310421047513778063246676"
        "89261670696623633820136378418383684178734361726757"
        "28112879812849979408065481931592621691275889832738"
        "44274228917432520321923589422876796487670272189318"
        "47451445736001306439091167216856844588711603153276"
        "70386486105843025439939619828917593665686757934951"
        "62176457141856560629502157223196586755079324193331"
        "64906352462741904929101432445813822663347944758178"
        "92575867718337217661963751590579239728245598838407"
        "58203565325359399008402633568948830189458628227828"
        "80181199384826282014278194139940567587151170094390"
        "35398664372827112653829987240784473053190104293586"
        "86515506006295864861532075273371959191420517255829"
        "71693888707715466499115593487603532921714970056938"
        "54370070576826684624621495650076471787294438377604"
        "53282654108756828443191190634694037855217779295145"
        "36123272525000296071075082563815656710885258350721"
        "45876576172410976447339110607218265236877223636045"
        "17423706905851860660448207621209813287860733969412"
        "81142660418086830619328460811191061556940512689692"
        "51934325451728388641918047049293215058642563049483"
        "62467221648435076201727918039944693004732956340691"
        "15732444386908125794514089057706229429197107928209"
        "55037687525678773091862540744969844508330393682126"
        "18336384825330154686196124348767681297534375946515"
        "80386287592878490201521685554828717201219257766954"
        "78182833757993103614740356856449095527097864797581"
        "16726320100436897842553539920931837441497806860984"
        "48403098129077791799088218795327364475675590848030"
        "87086987551392711854517078544161852424320693150332"
        "59959406895756536782107074926966537676326235447210"
        "69793950679652694742597709739166693763042633987085"
        "41052684708299085211399427365734116182760315001271"
        "65378607361501080857009149939512557028198746004375"
        "35829035317434717326932123578154982629742552737307"
        "94953759765105305946966067683156574377167401875275"
        "88902802571733229619176668713819931811048770190271"
        "25267680276078003013678680992525463401061632866526"
        "36270218540497705585629946580636237993140746255962"
        "24074486908231174977792365466257246923322810917141"
        "91430288197103288597806669760892938638285025333403"
        "34413065578016127815921815005561868836468420090470"
        "23053081172816430487623791969842487255036638784583"
        "11487696932154902810424020138335124462181441773470"
        "63783299490636259666498587618221225225512486764533"
        "67720186971698544312419572409913959008952310058822"
        "95548255300263520781532296796249481641953868218774"
        "76085327132285723110424803456124867697064507995236"
        "37774242535411291684276865538926205024910326572967"
        "23701913275725675285653248258265463092207058596522"
        "29798860272258331913126375147341994889534765745501"
        "18495701454879288984856827726077713721403798879715"
        "38298203783031473527721580348144513491373226651381"
        "34829543829199918180278916522431027392251122869539"
        "40957953066405232632538044100059654939159879593635"
        "29746152185502371307642255121183693803580388584903"
        "41698116222072977186158236678424689157993532961922"
        "62467957194401269043877107275048102390895523597457"
        "23189706772547915061505504953922979530901129967519"
        "86188088225875314529584099251203829009407770775672"
        "11306739708304724483816533873502340845647058077308"
        "82959174767140363198008187129011875491310547126581"
        "97623331044818386269515456334926366572897563400500"
        "42846280183517070527831839425882145521227251250327"
        "55121603546981200581762165212827652751691296897789"
        "32238195734329339946437501907836945765883352399886"
        "75506164965184775180738168837861091527357929701337"
        "62177842752192623401942399639168044983993173312731"
        "32924185707147349566916674687634660915035914677504"
        "99518671430235219628894890102423325116913619626622"
        "73267460800591547471830798392868535206946944540724"
        "76841822524674417161514036427982273348055556214818"
        "97142617910342598647204516893989422179826088076852"
        "87783646182799346313767754307809363333018982642090"
        "10848802521674670883215120185883543223812876952786"
        "71329612474782464538636993009049310363619763878039"
        "62184073572399794223406235393808339651327408011116"
        "66627891981488087797941876876144230030984490851411"
        "60661826293682836764744779239180335110989069790714"
        "85786944089552990653640447425576083659976645795096"
        "66024396409905389607120198219976047599490197230297"
        "64913982680032973156037120041377903785566085089252"
        "16730939319872750275468906903707539413042652315011"
        "94809377245048795150954100921645863754710598436791"
        "78639167021187492431995700641917969777599028300699"
        "15368713711936614952811305876380278410754449733078"
        "40789923115535562561142322423255033685442488917353"
        "44889911501440648020369068063960672322193204149535"
        "41503128880339536053299340368006977710650566631954"
        "81234880673210146739058568557934581403627822703280"
        "82616570773948327592232845941706525094512325230608"
        "22918802058777319719839450180888072429661980811197"
        "77158542502016545090413245809786882778948721859617"
        "72107838435069186155435662884062257473692284509516"
        "20849603980134001723930671666823555245252804609722"
        "53503534226472524250874054075591789781264330331690";
        int64_t u = 0;

        for( int i = 0 ; i < 100 ; i++ )
        {
            int64_t c = 0;
            char *p = s+i*50;
            for( int j = 0 ; j < 11 ; j++ )
            {
                c *= 10;
                c += *(p+j) - '0';
            }
            u += c;
        }

        int len = 0;
        int64_t v = 1;
        while( u/v ) len++, v *= 10;
        u /= v/10000000000;
        printf("Ans = %jd\n", u);
}

int main()
{
    clock_t t;

    t = clock();
    solve1();
    printf("Elapsed time is %.3f seconds \n", (float)(clock() - t) / CLOCKS_PER_SEC);

    return 0;
}