/in/lab5.c: In function 'infixToPostfix':
/in/lab5.c:59:13: warning: implicit declaration of function 'isdigit' [-Wimplicit-function-declaration]
         if (isdigit(infix[i]) || isalpha(infix[i])) {
             ^~~~~~~
/in/lab5.c:59:34: warning: implicit declaration of function 'isalpha' [-Wimplicit-function-declaration]
         if (isdigit(infix[i]) || isalpha(infix[i])) {
                                  ^~~~~~~