Fenwick tree lca

#include <iostream>
#include <vector>

using ll = long long;

int const nmax = 200000;
int parent[1 + nmax];
int fen[1 + nmax];
int level[1 + nmax];
inline int LSB(int x) {
    return x &-x;
}

inline int anc(int node, int k) {
    int r = level[node];
    int l = r - k;
    if (l < 1)return -1;
    while (l < r) {
      int nr = r - LSB(r);
      bool exceed = nr < l;
      node = exceed ? parent[node] : fen[node];
      r = exceed ? r - 1 : nr;
    }
    return node;
}

inline int lca(int a, int b) {
    if (level[a]>level[b]) {
        std::swap(a,b);
    }
    b=anc(b,level[b]-level[a]);
    int r=level[a];
    while(a!=b) {
        if (fen[a]==fen[b]){
            r--;
            a=parent[a];
            b=parent[b];
        } else {
            r-=r&-r;
            a=fen[a];
            b=fen[b];
        }
    }
    return a;
}

int main() {
  int n, q;
  std::cin >> n >> q;
  for(int i = 2; i <= n; i++)
    std::cin >> parent[i];
  level[1] = 1;
  for(int i = 2; i <= n; i++) {
    level[i] = level[parent[i]] + 1;
    int sz=LSB(level[i]);
    int p=parent[i];
    while(sz>>=1){
        p=fen[p];
    }
    fen[i]=p;
  }

  while(q--) {
    int a, b;
    std::cin >> a >> b;

    std::cout << lca(a, b) << '\n';
  }
  return 0;
}