《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 = 1): total_capacity(_total_capacity) {

        msize[0] = 0;

        msize[1] = 0;

        msize[2] = 0;

        

        mcapacity[0] = mcapacity[1] = mcapacity[2] = total_capacity / 3;

        mcapacity[0] += (total_capacity % 3 != 0 ? 1 : 0);

        mcapacity[1] += (total_capacity % 3 == 2 ? 1 : 0);

        

        offset[0] = 0;

        offset[1] = offset[0] + mcapacity[0];

        offset[2] = offset[1] + mcapacity[1];

        

        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] == 0) {

            return;

        }

        

        --msize[idx];

    }

    

    T top(int idx) {

        if (msize[idx] == 0) {

            return mdata[-1];

        }

        

        return mdata[offset[idx] + msize[idx] - 1];

    }

    

    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[3];

    // capacities of the three stacks

    size_t mcapacity[3];

    // sizes of the three stacks

    size_t msize[3];

    // the data in the stacks

    vector<T> mdata;

};



int main()

{

    int n;

    size_t idx;

    int val;

    char str[100];

    

    scanf("%d", &n);

    ThreeStack<int> ts(n);

    while (scanf("%s", str) == 1) {

        if (strcmp(str, "end") == 0) {

            break;

        } else if (strcmp(str, "push") == 0) {

            scanf("%u%d", &idx, &val);

            ts.push(idx, val);

        } else if (strcmp(str, "pop") == 0) {

            scanf("%u", &idx);

            ts.pop(idx);

        } else if (strcmp(str, "top") == 0) {

            scanf("%u", &idx);

            printf("top[%u] = %d\n", idx, ts.top(idx));

        } else if (strcmp(str, "size") == 0) {

            scanf("%u", &idx);

            printf("size[%u] = %u\n", idx, ts.size(idx));

        }

    }

    

    return 0;

}