HDU 1754（线段树）

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1754

区间求和问题~~~

代码：

#include <stdio.h>

#define N 200002

#define M 5002

#define L(x) ((x)<<1)

#define R(x) ((x)<<1|1)



typedef struct

{

    int lson, rson;

    int val;

}   seg_tree;

seg_tree s[N<<2];

int num[N];



int build(int left, int right, int idx)

{

    s[idx].lson = left;

    s[idx].rson = right;

    if(left == right)

    {

        return s[idx].val = num[left];

    }

    int mid = (s[idx].lson + s[idx].rson) / 2;

    int a, b;

    return s[idx].val = ((a = build(left, mid, L(idx))) > (b = build(mid + 1, right, R(idx))) ? a : b);

}



void update(int id, int val, int idx)

{

    if(s[idx].lson == s[idx].rson)

    {

        s[idx].val = val;

        return;

    }

    int mid = (s[idx].lson + s[idx].rson) / 2;

    if(id <= mid)

    {

        update(id, val, L(idx));

    }

    else

    {

        update(id, val, R(idx));

    }

    s[idx].val =  s[L(idx)].val > s[R(idx)].val ? s[L(idx)].val : s[R(idx)].val;

}



int query(int left, int right, int idx)

{

    if(left == s[idx].lson && s[idx].rson == right)

    {

        return s[idx].val;

    }

    int mid = (s[idx].lson + s[idx].rson) / 2;

    if(right <= mid)

    {

        return query(left, right, L(idx));

    }

    else if(left > mid)

    {

        return query(left, right, R(idx));

    }

    else

    {

        int a, b;

        return  (a = query(left, mid, L(idx))) > (b = query(mid + 1, right, R(idx))) ? a : b;

    }

}



int main()

{

    int n, m;

    int i, j;

    while(scanf("%d%d", &n, &m) != EOF)

    {

        for(i = 1; i <= n; i++)

        {

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

        }

        build(1, n, 1);

        for(j = 0; j < m; j++)

        {

            char c;

            int a, b;

            scanf("%s %d %d", &c, &a, &b);

            switch(c)

            {

                case 'U':

                    update(a, b, 1);

                    break;

                case 'Q':

                    printf("%d\n", query(a, b, 1));

                    break;

            }

        }

    }

}