6栈和队列

#define PUSH(n)                 do {stack[p++] = n;} while(0)
#define POP(n)                  do {n = stack[--p];} while(0)
#define STACK_TOP               stack[p-1]
#define STACK_ISNOT_EMPTY       (p != 0)             

char* removeDuplicates(char* s) {

    char *stack = (char*)malloc(sizeof(char)*strlen(s)+1);
    int p = 0;
    int i = 0;
    char c;

    while(s[i] != '\0')
    {
        if (STACK_ISNOT_EMPTY && STACK_TOP == s[i])
            POP(c);
        else
            PUSH(s[i]);
        i++;
    }

    PUSH('\0');
    return stack;
}

typedef struct {
    int a[10000];
    int p;
} stack_t;

stack_t stack;

#define PUSH(n)     stack.a[stack.p++] = n
#define POP(n)      n = stack.a[--stack.p]
#define STACK_INIT  stack.p = 0

int evalRPN(char** tokens, int tokensSize) {

    STACK_INIT;

    for (int i = 0; i<tokensSize; i++)
    {
        if ( (tokens[i][0]<'0') && (tokens[i][1] == '\0'))
        {
            POP(int b);
            POP(int a);
            
            int c;

            switch(tokens[i][0])
            {
                case '+' : c = a + b; break;
                case '-' : c = a - b; break;
                case '*' : c = a * b; break;
                case '/' : c = a / b; break;
                default : ;
            }

            PUSH(c);
        }
        else
        {
            PUSH(atoi(tokens[i]));
        }
    } 

    POP(int result);
    return result;
}