#include <map>
#include <set>
#include <ctime>
#include <cmath>
#include <queue>
#include <stack>
#include <vector>
#include <string>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <functional>
using namespace std;
inline int read(){int now=0;register char c=getchar();for(;!isdigit(c);c=getchar());
for(;isdigit(c);now=now*10+c-'0',c=getchar());return now;}
#define For(i, x, y) for(int i=x;i<=y;i++)
#define _For(i, x, y) for(int i=x;i>=y;i--)
#define Mem(f, x) memset(f,x,sizeof(f))
#define Sca(x) scanf("%d", &x)
#define Sca2(x,y) scanf("%d%d",&x,&y)
#define Sca3(x,y,z) scanf("%d%d%d",&x,&y,&z)
#define Scl(x) scanf("%lld",&x);
#define Pri(x) printf("%d\n", x)
#define Prl(x) printf("%lld\n",x);
#define CLR(u) for(int i=0;i<=N;i++)u[i].clear();
#define LL long long
#define ULL unsigned long long
#define mp make_pair
#define PII pair<int,int>
#define PIL pair<int,long long>
#define PLL pair<long long,long long>
#define pb push_back
#define fi first
#define se second
typedef vector<int> VI;
const double eps = 1e-9;
const int maxn = 2e5 + 10;
const int maxm = 1e6 + 10;
const int INF = 0x3f3f3f3f;
const int mod = 1e9 + 7;
int N,Q;
LL a[maxn];
LL tree[maxn];
void add(int x,int t){for(;x <= N ; x += x & -x) tree[x] += t;}
LL getsum(int x){LL ans = 0; for(;x > 0 ; x -= x & -x) ans += tree[x]; return ans;}
void solve(){if(a[1] == 0) return (void)puts("1");int x = 1;while(x < N){LL sum = getsum(x) * 2;if(getsum(x + 1) == sum) return (void)printf("%d\n",x + 1);int ans = x,l = x + 1,r = N;while(l <= r){int m = (l + r) >> 1;if(getsum(m) < sum){ans = m;l = m + 1;}else{r = m - 1;}}if(ans > N) break;x = (x == ans)?ans + 1:ans;}puts("-1");
}
int main()
{Sca2(N,Q);for(int i = 1; i <= N ; i ++){Scl(a[i]); add(i,a[i]);} while(Q--){int x; LL p;scanf("%d%lld",&x,&p);add(x,p - a[x]); a[x] = p;solve();}return 0;
}

思路2

#include <map>
#include <set>
#include <ctime>
#include <cmath>
#include <queue>
#include <stack>
#include <vector>
#include <string>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <functional>
using namespace std;
inline int read(){int now=0;register char c=getchar();for(;!isdigit(c);c=getchar());
for(;isdigit(c);now=now*10+c-'0',c=getchar());return now;}
#define For(i, x, y) for(int i=x;i<=y;i++)
#define _For(i, x, y) for(int i=x;i>=y;i--)
#define Mem(f, x) memset(f,x,sizeof(f))
#define Sca(x) scanf("%d", &x)
#define Sca2(x,y) scanf("%d%d",&x,&y)
#define Sca3(x,y,z) scanf("%d%d%d",&x,&y,&z)
#define Scl(x) scanf("%lld",&x);
#define Pri(x) printf("%d\n", x)
#define Prl(x) printf("%lld\n",x);
#define CLR(u) for(int i=0;i<=N;i++)u[i].clear();
#define LL long long
#define ULL unsigned long long
#define mp make_pair
#define PII pair<int,int>
#define PIL pair<int,long long>
#define PLL pair<long long,long long>
#define pb push_back
#define fi first
#define se second
typedef vector<int> VI;
const double eps = 1e-9;
const int maxn = 2e5 + 10;
const int maxm = 1e6 + 10;
const int INF = 0x3f3f3f3f;
const int mod = 1e9 + 7;
int N,Q;
LL fa[maxn],a[maxn];
void add(int p,int t){for(;p <= N ; p += p & -p) fa[p] += t;
}
LL getsum(int p){LL ans = 0;for(;p > 0; p -= p & -p) ans += fa[p];return ans;
}
struct Node{int pos;LL MAX;Node(int pos = 0,LL MAX = 0):pos(pos),MAX(MAX) {}
};
Node operator + (Node a,Node b){if(a.MAX >= b.MAX) return a;return b;
}
Node operator - (Node a,Node b){if(a.pos >= b.pos) return b;return a;
}
struct Tree{int l,r;Node MAX;
}tree[maxn << 2];
void Build(int t,int l,int r){tree[t].l = l; tree[t].r = r;if(l == r){tree[t].MAX.MAX = a[l];tree[t].MAX.pos = l;return;}int m = (l + r) >> 1;Build(t << 1,l,m); Build(t << 1 | 1,m + 1,r);tree[t].MAX = tree[t << 1].MAX + tree[t << 1 | 1].MAX;
}
void update(int t,int pos,LL val){if(tree[t].l == tree[t].r){tree[t].MAX.MAX = val;return;}int m = (tree[t].l + tree[t].r) >> 1;if(pos <= m) update(t << 1,pos,val);else update(t << 1 | 1,pos,val);tree[t].MAX = tree[t << 1].MAX + tree[t << 1 | 1].MAX;
}
Node query(int t,int l,int r,LL sum){if(l <= tree[t].l && tree[t].r <= r){if(tree[t].MAX.MAX < sum) return Node(INF,1e18);if(tree[t].l == tree[t].r) return tree[t].MAX;int m = (tree[t].l + tree[t].r) >> 1;if(tree[t << 1].MAX.MAX >= sum) return query(t << 1,l,m,sum);return query(t << 1 | 1,m + 1,r,sum);}int m = (tree[t].l + tree[t].r) >> 1;if(r <= m) return query(t << 1,l,r,sum);else if(l > m) return query(t << 1 | 1,l,r,sum);else return query(t << 1,l,m,sum) - query(t << 1 | 1,m + 1,r,sum);
}int solve(){if(getsum(1) == 0) return 1;int s = 1;while(s < N){LL now = getsum(s);Node MAX = query(1,s + 1,N,now);if(MAX.pos == INF) return -1;if(getsum(MAX.pos - 1) == MAX.MAX) return MAX.pos;s = MAX.pos;}return -1;
}
int main()
{Sca2(N,Q);for(int i = 1; i <= N ; i ++){Scl(a[i]);add(i,a[i]);} Build(1,1,N);while(Q--){int x; LL p;scanf("%d%lld",&x,&p);update(1,x,p);add(x,p - a[x]); a[x] = p;Pri(solve());        }return 0;
}