HDU1754 线段树模板题

 1 #include<iostream>
 2 #include<string.h>
 3 using namespace std;
 4 
 5 const int N = 200005;
 6 int grade[N];
 7 int tree[N<<2];                    //这里建立的树的数组大小需要是N的4倍 否则不够用 
 8 int n, m;
 9 
10 int max(int a, int b){
11     if(a > b) return a;
12     else return b;
13 }
14 
15 void build_tree(int start, int end, int node){                    //线段树的建立 
16     if(start == end){
17         tree[node] = grade[start];
18     }else{
19         int mid = (start + end) / 2;
20         int left_node = node*2;
21         int right_node = node*2+1;
22         
23         build_tree(start, mid, left_node);
24         build_tree(mid+1, end, right_node);
25         tree[node] = max(tree[left_node], tree[right_node]);
26     }
27 }
28 
29 void update_tree(int start, int end, int node, int index, int value){    //单点更新值 
30     if(start == end){
31         tree[node] = value;
32     }else{
33         int mid = (start + end) / 2;
34         int left_node = node*2;
35         int right_node = node*2+1;
36         
37         if(index <= mid)
38             update_tree(start, mid, left_node, index, value);
39         else
40             update_tree(mid+1, end, right_node, index, value);
41         tree[node] = max(tree[left_node], tree[right_node]);
42     }
43 }
44 
45 int search_tree(int start, int end, int node, int l, int r){        //区间查询最大值 
46     if(l > end || r < start){
47         return 0;
48     }else if(l <= start && r >= end){
49         return tree[node];
50     }else if(start == end){　　　　　　//这里的个递归出口放在后面是有原因的，有可能存在start==end 但是l和r根本和start end没有交集的情况，所以先判去了后者
51         return tree[node];
52     }
53     int mid = (start + end) / 2;
54     int left_node = node*2;
55     int right_node = node*2+1;
56     
57     int left_max = search_tree(start, mid, left_node, l, r);
58     int right_max = search_tree(mid+1, end, right_node, l, r);
59     int ans = max(left_max, right_max);
60     return ans;
61 }
62 
63 int main(){
64     while(scanf("%d%d", &n, &m) != EOF){
65         for(int i = 1; i <= n; i++)
66             scanf("%d", &grade[i]);
67         build_tree(1, n, 1);    
68         for(int i = 1; i <= m; i++){
69             char c;
70             int a, b;
71             scanf(" %c %d %d", &c, &a, &b);            //对于c语言的输入字符在%c之前需要一个空格，否则c就读取不到需要的字符了 
72             if(c == ‘U‘) update_tree(1, n, 1, a, b);
73             else{
74                 int ans = search_tree(1, n, 1, a, b);
75                 printf("%d\n", ans);
76             }
77         }
78     }    
79     return 0;
80 }