线段树（自敲：建树，查找最大值，更新结点值）

HDU1754

#include <bits/stdc++.h>

using namespace std;

const int MaxSIZE = 2e6 + 10;

typedef struct {
    int Max ;
    int left, right ;
} NODE ;

int     n, m ;
int     num [MaxSIZE] ;
NODE    tree[MaxSIZE * 20] ;


int build (int root, int left, int right) {
    int mid ;

    tree[root].left     = left ;
    tree[root].right    = right ;

    if (left == right) {
        return tree[root].Max = num[left] ;
    }
    mid = (left + right) / 2 ;


    int a, b ;
    a = build (2 * root, left, mid) ;
    b = build (2 * root + 1, mid + 1, right) ;

    return tree[root].Max = max (a, b) ;
}

int Find (int root, int left, int right) {
    int mid ;

    if (tree[root].left > right || tree[root].right < left)
        return 0 ;

    if (left <= tree[root].left && tree[root].right <= right)
        return tree[root].Max ;

    int a, b ;
    a = Find (2 * root, left, right) ;
    b = Find (2 * root + 1, left, right) ;

    return max (a, b) ;
}


int update (int root, int pos, int val) {

    if (pos < tree[root].left || tree[root].right < pos)
        return tree[root].Max ;

    if (tree[root].left == pos && tree[root].right == pos)
        return tree[root].Max = val ;

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

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

    return tree[root].Max ;
}


int main() {
    int n, ask_n;
    while (~scanf("%d%d", &n, &ask_n)) {
    //while (cin >> n >> ask_n) {
        memset(num, 0, sizeof(num));

        for (int i = 1; i <= n; ++ i) {
            //cin >> num[i];
            scanf("%d", &num[i]);
        }

        build (1, 1, n);

        char op; int x, y;

        for (int j = 0; j < ask_n; ++ j) {
            getchar();
            scanf("%c%d%d", &op, &x, &y);
            //cin >> op >> x >> y;
            if (op == 'Q') {
                int ans = Find (1, x, y);
                cout << ans << endl;
            } else if (op == 'U') {
                num[x] = y;
                update (1, x, y);
            }
        }
    }
    return 0;
}