63 lines
1.3 KiB
C
63 lines
1.3 KiB
C
/* Find the unique positive integer whose square has the form 1_2_3_4_5_6_7_8_9_0,
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* where each “_” is a single digit.*/
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#define _POSIX_C_SOURCE 199309L
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#include <stdio.h>
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#include <stdlib.h>
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#include <time.h>
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int main(int argc, char **argv)
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{
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int i, found = 0;
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/* Since the square of n has 19 digits, n must be at least 10^9.*/
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long int n = 1e9, p;
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double elapsed;
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struct timespec start, end;
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clock_gettime(CLOCK_MONOTONIC, &start);
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while(!found)
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{
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/* If the square on n ends with 10, n must be divisible by 10.*/
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n += 10;
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p = n * n;
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/* A square divisible by 10 is also divisible by 100.*/
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if(p % 100 != 0)
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{
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continue;
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}
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/* Check if the digits of the square correspond to the given pattern.*/
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i = 9;
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p /= 100;
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while(p > 0)
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{
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if(p % 10 != i)
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{
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break;
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}
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p /= 100;
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i--;
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}
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if(p == 0)
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{
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found = 1;
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}
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}
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clock_gettime(CLOCK_MONOTONIC, &end);
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elapsed = (end.tv_sec - start.tv_sec) + (double)(end.tv_nsec - start.tv_nsec) / 1000000000;
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printf("Project Euler, Problem 206\n");
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printf("Answer: %ld\n", n);
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printf("Elapsed time: %.9lf seconds\n", elapsed);
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return 0;
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}
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