程序代码：

/* 先统计， 再遍历*/
#include <stdio.h>
#include <stdlib.h>

#define NO_OF_CHARS 256

int *getCharCountArray(char *);

char getIndexOfMax( int *, int);

char getMaxOccuringChar(char *str);

int main(void) {

    char str[] = "sample string";

    printf("%c\n", getMaxOccuringChar(str));

    return 0;
}

char getMaxOccuringChar(char *str) {

    int *count = getCharCountArray(str);

    return getIndexOfMax(count, NO_OF_CHARS);

}

int *getCharCountArray(char *str) {

    int *count = (int *)calloc(NO_OF_CHARS, sizeof(int));

    int i;

    for(i = 0; *(str+i); i++) 

        count[*(str+i)]++;

    return count;
}

char getIndexOfMax(int ar[], int ar_size) {

    int i;

    int max_index = 0;

    for( i = 1; i < ar_size; i++) 

        if(ar[i] > ar[max_index])

            max_index = i;

    free(ar);
    
    ar = NULL;

    return max_index;
}