pat 1057. Stack (30)

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利用树状数组来做，树状节点储存该区间内的数字个数
树状数组每个节点x负责的区间为[x - 2^k + 1,x]
其中k为x用二进制表示中末尾0的个数

#include<stdio.h>#include<stack>#include<algorithm>#include<string.h>using namespace std;#define SIZE 100001struct key{    int num;    int flag;};class TreeArr{private:    int a[SIZE];    int lowbit(int x){        return x&(-x);    }public:    TreeArr(){        memset(a, 0, sizeof(a));    }    void update(int num, int value){        while (num <= SIZE){            a[num] += value;            num += lowbit(num);        }    }    int sum(int pos){        int value(0);        while (pos>0){            value += a[pos];            pos -= lowbit(pos);        }        return value;    }    int find(int num, int begin = 0, int end = SIZE){        if (begin == end)            return begin;        int mid = (begin + end) / 2;        if (sum(mid) >= num){            return find(num, begin, mid);        }        else return find(num, mid+1, end);    }};int main(){    stack<int>s;    TreeArr tree;    freopen("1.in", "r", stdin);    int n;    scanf("%d", &n);    while (n--){        char cmd[20];        scanf("%s", cmd);        if (strcmp(cmd, "Pop") == 0){            if (!s.size())                printf("Invalid\n");            else{                printf("%d\n", s.top());                tree.update(s.top(), -1);                s.pop();            }        }        else if (!strcmp(cmd, "Push")){            int key;            scanf("%d", &key);            s.push(key);            tree.update(key, 1);        }        else if (!strcmp(cmd, "PeekMedian")){            if (!s.size())                printf("Invalid\n");            else{                printf("%d\n", tree.find((s.size()+1) / 2));            }        }    }    return 0;}

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