HDU 1754 线段树入门解题报告

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题意：给定区间,每个人的成绩, Q次询问,求每次询问区间中的最大值

思路：构造线段树

代码：

#include<stdio.h>
#include<algorithm>
#include<string>
#include<iostream>
using namespace std;

int n, m, ans;
string str;

struct Tree
{
    int left, right, maxx;
}tree[ * ];  //线段树开4倍空间 

void build(int left, int right, int k)
{
    tree[k].left = left, tree[k].right = right;
    if(left == right)
    {
        scanf("%d", &tree[k].maxx);
        return ;
    }
    int mid = (left + right) / ;
    build(left, mid,  * k);
    build(mid + , right,  * k + );
    tree[k].maxx = max(tree[ * k].maxx, tree[ * k + ].maxx); //向上回溯最大值记录
}

void update(int find_node, int up_num, int k)//单点修改
{
    if(tree[k].left == find_node && tree[k].right == find_node)
    {
        tree[k].maxx = up_num;
        return ;
    }
    int mid = (tree[k].left + tree[k].right) / ;
    if(find_node <= mid)
        update(find_node, up_num,  * k);
    else
        update(find_node, up_num,  * k + );  //修改过后回溯修改最大值
    tree[k].maxx = max(tree[ * k].maxx, tree[ * k + ].maxx);
}

void query(int find_left, int find_right, int k)//区间查询取最大值
{
    if(find_left == tree[k].left && find_right == tree[k].right)
    {
        ans = max(ans, tree[k].maxx);
        return ;
    }
    int mid = (tree[k].left + tree[k].right) / ;
    if(find_right <= mid)
        query(find_left, find_right,  * k);
    else if(find_left > mid)
        query(find_left, find_right,  * k + );
    else
    {
        query(find_left, mid,  * k);
        query(mid + , find_right,  * k + );
    }
}

int main()
{
    int a, b;
    while(scanf("%d%d", &n, &m)!=EOF)
    {
        build(, n, );
        for(int i = ; i <= m; i ++)
        {
            cin >> str;
            if(str == "U")
            {
                scanf("%d%d", &a, &b);
                update(a, b, );
            }
            else
            {
                ans = -;
                scanf("%d%d", &a, &b);
                query(a, b, );
                printf("%d\n", ans);
            }
        }
    }
    return ;
}