PAT(A) - 1057. Stack (30)

Stack is one of the most fundamental data structures, which is based on the principle of Last In First Out (LIFO). The basic operations include Push (inserting an element onto the top position) and Pop (deleting the top element). Now you are supposed to implement a stack with an extra operation: PeekMedian -- return the median value of all the elements in the stack. With N elements, the median value is defined to be the (N/2)-th smallest element if N is even, or ((N+1)/2)-th if N is odd.

Input Specification:

Each input file contains one test case. For each case, the first line contains a positive integer N (<= 10⁵). Then N lines follow, each contains a command in one of the following 3 formats:

Push key
Pop
PeekMedian

where key is a positive integer no more than 10⁵.

Output Specification:

For each Push command, insert key into the stack and output nothing. For each Pop or PeekMedian command, print in a line the corresponding returned value. If the command is invalid, print "Invalid" instead.

Sample Input:

17PopPeekMedianPush 3PeekMedianPush 2PeekMedianPush 1PeekMedianPopPopPush 5Push 4PeekMedianPopPopPopPop

Sample Output:

InvalidInvalid322124453Invalid

思路分析：直接暴力会超时，我以前参考别人的代码，利用树状数组做的。今天看了晴神的书，用的分块思想，好神奇呀~

#include <cstdio>#include <stack>#include <cmath>#include <string.h>#define MAX 100000 + 10using namespace std;int block[316];int table[MAX];int sqr = ( int )sqrt( MAX );stack<int> s;using namespace std;void Push( int num ) {    s.push( num );    block[num / sqr]++;    table[num]++;}void Pop() {    if( s.empty() ) {        printf( "Invalid\n" );        return;    }    int top = s.top();    s.pop();    table[top]--;    block[top / sqr]--;    printf( "%d\n", top );}void PeekMedian() {    if( s.empty() ) {        printf( "Invalid\n" );        return;    }    int k = s.size();    if( k % 2 == 1 ) k = ( k + 1 ) / 2;    else k = k / 2;    int sum = 0;    int id = 0;    while( sum + block[id] < k ) {        sum = sum + block[id];        id++;    }    int num = id * sqr;    while( sum + table[num] < k ) {        sum += table[num];        num++;    }    printf( "%d\n", num );}int main() {    //freopen( "123.txt", "r", stdin );    int n;    scanf( "%d", &n );    char op[20];    int num;    for( int i = 0; i < n; i++ ) {        scanf( "%s", op );        if( !strcmp( op, "Push" ) ) {            scanf( "%d", &num );            Push( num );        }        else if( !strcmp( op, "Pop" ) ) {            Pop();        }        else if( !strcmp( op, "PeekMedian" ) ) {            PeekMedian();        }    }    return 0;}