《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
", idx, ts.top(idx));
        } else if (strcmp(str, "size") == 0) {
            scanf("%u", &idx);
            printf("size[%u] = %u
", idx, ts.size(idx));
        }
    }
    
    return 0;
}