线段树（自敲：建树，查找最大值，更新结点值）

  1 #include <bits/stdc++.h>
  2 
  3 using namespace std;
  4 
  5 const int MaxSIZE = 2e6 + 10;
  6 
  7 typedef struct {
  8     int Max ;
  9     int left, right ;
 10 } NODE ;
 11 
 12 int     n, m ;
 13 int     num [MaxSIZE] ;
 14 NODE    tree[MaxSIZE * 20] ;
 15 
 16 
 17 int build (int root, int left, int right) {
 18     int mid ;
 19 
 20     tree[root].left     = left ;
 21     tree[root].right    = right ;
 22 
 23     if (left == right) {
 24         return tree[root].Max = num[left] ;
 25     }
 26     mid = (left + right) / 2 ;
 27 
 28 
 29     int a, b ;
 30     a = build (2 * root, left, mid) ;
 31     b = build (2 * root + 1, mid + 1, right) ;
 32 
 33     return tree[root].Max = max (a, b) ;
 34 }
 35 
 36 int Find (int root, int left, int right) {
 37     int mid ;
 38 
 39     if (tree[root].left > right || tree[root].right < left)
 40         return 0 ;
 41 
 42     if (left <= tree[root].left && tree[root].right <= right)
 43         return tree[root].Max ;
 44 
 45     int a, b ;
 46     a = Find (2 * root, left, right) ;
 47     b = Find (2 * root + 1, left, right) ;
 48 
 49     return max (a, b) ;
 50 }
 51 
 52 
 53 int update (int root, int pos, int val) {
 54 
 55     if (pos < tree[root].left || tree[root].right < pos)
 56         return tree[root].Max ;
 57 
 58     if (tree[root].left == pos && tree[root].right == pos)
 59         return tree[root].Max = val ;
 60 
 61     int a, b ;
 62     a = update (2 * root, pos, val) ;
 63     b = update (2 * root + 1, pos, val) ;
 64 
 65     tree[root].Max = max (a, b) ;
 66 
 67     return tree[root].Max ;
 68 }
 69 
 70 
 71 int main() {
 72     int n, ask_n;
 73     while (~scanf("%d%d", &n, &ask_n)) {
 74     //while (cin >> n >> ask_n) {
 75         memset(num, 0, sizeof(num));
 76 
 77         for (int i = 1; i <= n; ++ i) {
 78             //cin >> num[i];
 79             scanf("%d", &num[i]);
 80         }
 81 
 82         build (1, 1, n);
 83 
 84         char op; int x, y;
 85 
 86         for (int j = 0; j < ask_n; ++ j) {
 87             getchar();
 88             scanf("%c%d%d", &op, &x, &y);
 89             //cin >> op >> x >> y;
 90             if (op == ‘Q‘) {
 91                 int ans = Find (1, x, y);
 92                 cout << ans << endl;
 93             } else if (op == ‘U‘) {
 94                 num[x] = y;
 95                 update (1, x, y);
 96             }
 97         }
 98     }
 99     return 0;
100 }