《Cracking the Coding Interview》——第3章：栈和队列——题目1

2014-03-18 03:19

题目：用一个数组实现3个栈。

解法：

　　首先我想过让三个栈动态决定长度。要么左右各一个向中间靠拢，要么三个穿插着，后来都觉得实现起来太复杂，而且思路总有各种功能缺陷，会导致额外的时间或空间复杂度。所以，还是三等分成固定大小吧。好写又好用。

代码：

// 3.1 Use an array to implement three stacks.
// three fixed-length stacks
#include <cstdio>
#include <cstring>
#include <vector>
using namespace std;

template <class T>
class ThreeStack {
public:
    ThreeStack(size_t _total_capacity = ): total_capacity(_total_capacity) {
        msize[] = ;
        msize[] = ;
        msize[] = ;

        mcapacity[] = mcapacity[] = mcapacity[] = total_capacity / ;
        mcapacity[] += (total_capacity %  !=  ?  : );
        mcapacity[] += (total_capacity %  ==  ?  : );

        offset[] = ;
        offset[] = offset[] + mcapacity[];
        offset[] = offset[] + mcapacity[];

        mdata.resize(total_capacity);
    }

    ~ThreeStack() {
        mdata.clear();
    }

    void push(int idx, T val) {
        if (msize[idx] == mcapacity[idx]) {
            // this stack is full
            return;
        }

        mdata[offset[idx] + msize[idx]] = val;
        ++msize[idx];
    }

    void pop(int idx) {
        if (msize[idx] == ) {
            return;
        }

        --msize[idx];
    }

    T top(int idx) {
        if (msize[idx] == ) {
            return mdata[-];
        }

        return mdata[offset[idx] + msize[idx] - ];
    }

    size_t size(int idx) {
        return msize[idx];
    }
private:
    // total capacity of all stack
    size_t total_capacity;
    // starting offset for each stack
    size_t offset[];
    // capacities of the three stacks
    size_t mcapacity[];
    // sizes of the three stacks
    size_t msize[];
    // the data in the stacks
    vector<T> mdata;
};

int main()
{
    int n;
    size_t idx;
    int val;
    char str[];

    scanf("%d", &n);
    ThreeStack<int> ts(n);
    while (scanf("%s", str) == ) {
        if (strcmp(str, "end") == ) {
            break;
        } else if (strcmp(str, "push") == ) {
            scanf("%u%d", &idx, &val);
            ts.push(idx, val);
        } else if (strcmp(str, "pop") == ) {
            scanf("%u", &idx);
            ts.pop(idx);
        } else if (strcmp(str, "top") == ) {
            scanf("%u", &idx);
            printf("top[%u] = %d\n", idx, ts.top(idx));
        } else if (strcmp(str, "size") == ) {
            scanf("%u", &idx);
            printf("size[%u] = %u\n", idx, ts.size(idx));
        }
    }

    return ;
}