hdu1754线段树维护区间最大值

#include <iostream>

#include <cstdio>

using namespace std;

#define MAXN 200005

int N,M;

int grade[MAXN];

struct node

{

    int left;

    int right;

    int max;

}Tree[MAXN*20];

int Build(int start,int end,int root)

{

    Tree[root].left = start;

    Tree[root].right = end;

    if(start == end)

    {

        Tree[root].max = grade[start];

        return Tree[root].max;

    }

    int mid = (start + end)/2;

    int a = Build(start,mid,root*2);

    int b = Build(mid+1,end,(root*2)+1);

    return Tree[root].max = max(a,b);

}

int find(int start,int end,int root)

{

    if(start > Tree[root].right || end < Tree[root].left)

        return 0;

    if(start <= Tree[root].left && Tree[root].right <= end)

        return Tree[root].max;

    int a = find(start,end,root*2);

    int b = find(start,end,(root*2)+1);

    return max(a,b);

}

int update(int root,int pos,int value)

{

    if(pos < Tree[root].left || pos > Tree[root].right)

        return Tree[root].max;

    if(Tree[root].left == pos && Tree[root].right == pos)

        return Tree[root].max = value;

    int a = update(root*2,pos,value);

    int b = update((root*2)+1,pos,value);

    Tree[root].max = max(a,b);

    return Tree[root].max;

}

int main()

{

    while(scanf("%d%d",&N,&M)!=EOF)

    {

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

        {

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

        }

        Build(1,N,1);

        char ch;

        int A,B;

        while(M--)

        {

            getchar();

            scanf("%c%d%d",&ch,&A,&B);

            if(ch == 'Q')

                cout<<find(A,B,1)<<endl;

            else if(ch == 'U')

            {

                grade[A] = B;

                update(1,A,B);

            }

        }

    }

    return 0;

}