Template——Maximum problem of line segment tree (single point modification, interval query)

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

const int maxn = 1e5 + 10;

int pos[maxn];
struct tree {
    
    
	int l, r;
	int val;
}t[maxn * 4];
int ans = 0;

void Build(int i, int l, int r) {
    
    
	t[i].l = l;
	t[i].r = r;
	t[i].val = 0;
	if (l == r) {
    
    
		pos[l] = i; return;
	}
	int mid = l + r >> 1;
	Build(i << 1, l, mid);
	Build((i << 1) + 1, mid + 1, r);
}

void Update(int i) {
    
    
	if (i == 1) return;
	int root = i >> 1;
	int x = t[root << 1].val;
	int y = t[(root << 1) + 1].val;
	t[root].val = max(x, y);
	Update(root);
}

void Query(int i, int l, int r) {
    
    
	if (l == t[i].l && r == t[i].r) {
    
    
		ans = max(ans, t[i].val);
		return;
	}
	i <<= 1;
	if (l <= t[i].r) {
    
    
		if (r <= t[i].r) Query(i, l, r);
		else Query(i, l, t[i].r);
	}
	i++;
	if (r >= t[i].l) {
    
    
		if (l >= t[i].l) Query(i, l, r);
		else Query(i, t[i].l, r);
	}
}

int main() {
    
    

	int n, m;
	while (scanf("%d %d", &n, &m) != EOF) {
    
    
		Build(1, 1, n);
		for (int i = 1; i <= n; i++) {
    
    
			int val; scanf("%d", &val);
			t[pos[i]].val = val;
			Update(pos[i]);
		}

		while (m--) {
    
    
			string s;
			cin >> s;

			if (s[0] == 'Q') {
    
    
				int l, r;
				scanf("%d %d", &l, &r);
				ans = 0;
				Query(1, l, r);
				printf("%d\n", ans);
			}
			else {
    
    
				int i, val;
				scanf("%d %d", &i, &val);
				t[pos[i]].val = val;
				Update(pos[i]);
			}
		}
	}
	return 0;
}