LCA (Lowest Common Ancestor)

#include <bits/stdc++.h>
#define ll long long
#define SaveTime ios_base::sync_with_stdio(false), cin.tie(0);
using namespace std;

const int N = 3e5+7;
const int LOG = 20;
int up[N][LOG];
int depth[N];

vector<vector<int>> tree(N);

void dfs(int a) {
   for (int b : tree[a]) {
      depth[b] = depth[a]+1;
      up[b][0] = a;
      for (int j = 1; j < LOG; j++)
         up[b][j] = up[up[b][j-1]][j-1];
      dfs(b);
   }
}

int LCA(int a, int b) {
   if (depth[b] > depth[a])
      swap(a,b);
   int k = depth[a]-depth[b];
   for (int j = LOG-1; j >= 0; j--) {
      if (k & (1 << j))
         a = up[a][j];
   }
   if (a == b)
      return a;
   for (int j = LOG-1; j >= 0; j--) {
      if (up[a][j] != up[b][j]) {
         a = up[a][j];
         b = up[b][j];
      }
   }
   return up[a][0];
}

int main() {
   SaveTime
}

Resources :
https://youtu.be/oib-XsjFa-M
https://youtu.be/dOAxrhAUIhA
Problems :
https://codeforces.com/gym/101473/attachments (Problem J)