解题思路：就是用栈来模拟浏览器的前进、后退操作。

代码：

#include 
    
     struct stack {public:    stack() : top(0), bottom(0) {}    void push(const char *page);    void pop(char *page);    bool is_empty();    void clear();private:    void str_copy(char *des, const char *src);    char url[101][71];    int top, bottom;};void stack::str_copy(char *des, const char *src) {    while ((*des++ = *src++));}void stack::push(const char *page) {    str_copy(url[top++], page);}void stack::pop(char *page) {    str_copy(page, url[--top]);}bool stack::is_empty() {    return top == bottom;}void stack::clear() {    top = bottom = 0;}int main() {    char cmd[8];    char cur_page[71] = "http://www.acm.org/";    stack fw_stack, bw_stack;    while (scanf("%s", cmd) && cmd[0] != 'Q') {        switch (cmd[0]) {        case 'V':            bw_stack.push(cur_page);            fw_stack.clear();            scanf("%s", cur_page);            printf("%s\n", cur_page);            break;        case 'B':            if (bw_stack.is_empty()) {                printf("Ignored\n");            } else {                fw_stack.push(cur_page);                bw_stack.pop(cur_page);                printf("%s\n", cur_page);            }            break;        case 'F':            if (fw_stack.is_empty()) {                printf("Ignored\n");            } else {                bw_stack.push(cur_page);                fw_stack.pop(cur_page);                printf("%s\n", cur_page);            }            break;        }    }    return 0;}