Algorithm

HackerRank/Algorithm/Time Conversion

Problem Summary

Given a time in 12-hour AM/PM format, convert it to 24-hour time.

About the 12-hour clock:

https://en.wikipedia.org/wiki/12-hour_clock

Solution

Note that Midnight is 12:00:00AM on a 12-hour clock, and 00:00:00 on a 24-hour clock, and similarly, noon is 12:00:00PM on a 12-hour clock and 12:00:00 on a 24-hour clock.
Once we know this, the coding is trivial.

Code

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#include <iostream>
#include <cstring>
using namespace std;
string timeConversion(string s) {
    
    string t(s,0,8);
    int h = (s[0]-'0')*10 + s[1]-'0';
    
    if(s[8]=='A' && h==12)
        h=0;
    else
    if(s[8]=='P' && h<12)
        h+=12;
    t[0] = '0' + h/10;
    t[1] = '0' + h%10;
    return t;
}
int main() {
    string s;
    cin >> s;
    string result = timeConversion(s);
    cout << result << endl;
    return 0;
}