HDU 4747 Mex ( 线段树好题 + 思路 )

参考：http://www.cnblogs.com/oyking/p/3323306.html

相当不错的思路，膜拜之~

个人理解改日补充。

#include <cstdio>

#include <cstring>

#include <cstdlib>

#include <algorithm>



#define lson l, m, rt << 1

#define rson m + 1, r, rt << 1 | 1

#define lc rt << 1

#define rc rt << 1 | 1

#define LL long long int



using namespace std;



const int MAXN = 200010;



struct node

{

    int pos;

    int next;

};



LL sum[ MAXN << 2 ];

int maxi[ MAXN << 2 ];

int mini[ MAXN << 2 ];

int N;

int num[MAXN];

node D[ MAXN << 1 ];

int head[MAXN];

int EdgeN;



void AddEdge( int u, int v )

{

    D[EdgeN].pos = v;

    D[EdgeN].next = head[u];

    head[u] = EdgeN++;

    return;

}



void PushDown( int rt, int m )

{

    if ( maxi[rt] == mini[rt] )

    {

        maxi[lc] = mini[lc] = maxi[rt];

        sum[lc] = (LL)maxi[rt]*( m - ( m >> 1 ) );

        maxi[rc] = mini[rc] = mini[rt];

        sum[rc] = (LL)maxi[rt]*( m >> 1 );

    }

    return;

}



void PushUp( int rt )

{

    sum[rt] = sum[lc] + sum[rc];

    maxi[rt] = max( maxi[lc], maxi[rc] );

    mini[rt] = min( mini[lc], mini[rc] );

    return;

}



void build( int l, int r, int rt )

{

    if ( l == r )

    {

        sum[rt] = maxi[rt] = mini[rt] = N - l + 1;

        return;

    }

    int m = ( l + r ) >> 1;

    build( lson );

    build( rson );

    PushUp( rt );

    return;

}



void update( int L, int R, int val, int l, int r, int rt )

{

    if ( L <= l && r <= R && mini[rt] >= val )

    {

        sum[rt] = (LL)val*(r - l + 1);

        maxi[rt] = mini[rt] = val;

        return;

    }

    if ( l == r ) return;

    PushDown( rt, r - l + 1 );

    int m = ( l + r ) >> 1;

    if ( L <= m && maxi[lc] > val ) update( L, R, val, lson );

    if ( R > m && maxi[rc] > val )  update( L, R, val, rson );

    PushUp( rt );

    return;

}



int main()

{

    while ( scanf( "%d", &N ) == 1 && N )

    {

        for ( int i = 1; i <= N; ++i )

            scanf( "%d", &num[i] );

        memset( head, -1, sizeof(int)*(N+2) );

        EdgeN = 1;

        for ( int i = N; i > 0; --i )

        {

            if ( num[i] <= N )

                AddEdge( num[i], i );

        }

        build( 1, N, 1 );

        LL ans = 0;

        for ( int i = 0; i <= N && sum[1]; ++i )

        {

            int pre = 0;

            for ( int j = head[i]; j != -1; j = D[j].next )

            {

                update( pre + 1, D[j].pos, N - D[j].pos + 1, 1, N, 1 );

                pre = D[j].pos;

                //printf("num=%d pos=%d\n", i, D[j].pos );

            }

            update( pre + 1, N, 0, 1, N, 1 );

            ans += sum[1];

            //printf( "ans=%I64d\n", ans );

        }

        printf("%I64d\n", ans );

    }

    return 0;

}