HDU 1754 I Hate It【线段树】

   没想到竟然能一次AC，而且只花了20分钟不到。第一道线段树的题了，纪念一下。

　　考察线段树的基本操作:建树,区间最值查询和更新操作.

#include <iostream>

using namespace std;
const int MAX = 200005;
struct Tree
{
    int left; int right; int score;
}tr[MAX*3];
int n,m,s[MAX],a,b;
char cmd;

int max(int x,int y)
{
    if(x > y) return x;
    else return y;
}

void build(int l,int r,int now)
{
    tr[now].left = l; tr[now].right = r;

    if(l == r)
    {
        tr[now].score = s[l]; return;
    }

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

    build(l,mid,(now<<1));
    build(mid+1,r,(now<<1)+1);

    tr[now].score = max(tr[now<<1].score,tr[(now<<1)+1].score);
}

void modify(int l,int r,int now)
{
    if(l == r)    { tr[now].score = b; return; }

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

    if(a <= mid)
    {
        modify(l,mid,now<<1);
    }
    else if (a > mid)
    {
        modify(mid+1,r,(now<<1)+1);
    }
    tr[now].score = max(tr[now<<1].score,tr[(now<<1)+1].score);
}

int query(int sta,int end,int now)
{
    if(tr[now].left == sta && tr[now].right == end) return tr[now].score;

    int mid = (tr[now].left + tr[now].right)>>1;

    if(end <= mid) 
    {
        return query(sta,end,now<<1);
    }
    else if(sta > mid) 
    {
        return query(sta,end,(now<<1)+1);
    }
    else
    {
        int x = query(sta,mid,now<<1);
        int y = query(mid+1,end,(now<<1)+1);
        return max(x,y);
    }
}

int main()
{
    int i;

    while(scanf("%d %d",&n,&m) != EOF)
    {
        for(i = 1;i <= n;++i) scanf("%d",&s[i]);
        
        build(0,n,1);

        for(i = 0;i < m;++i)
        {
            cin>>cmd; scanf("%d %d",&a,&b);

            if(cmd == 'Q') printf("%d\n",query(a,b,1));
            else modify(0,n,1);
        }
    }

    return 0;
}