Add CSES Binary Lifting solution

CSES/Company Queries.cpp

@@ -0,0 +1,86 @@
+// https://cses.fi/problemset/task/1687/
+// O(q * log(n) + n*log(n))
+// Solution using Binary Lifting
+#include <bits/stdc++.h>
+using namespace std;
+
+int dfs(int root, vector<vector<int>>& tree, int d,
+vector<int>& depth) {
+    depth[root] = d;
+    int curr = d;
+    for(auto &adj : tree[root]) {
+        curr = max(curr, dfs(adj, tree, d + 1, depth));
+    }
+    return curr;
+}
+
+int getDepth(int root, vector<int>& par, vector<int>& depth) {
+    int n = par.size();
+    vector<vector<int>> tree(n);
+    for(int i = 1; i < n; ++i) {
+        tree[par[i]].push_back(i);
+    }
+    return dfs(root, tree, 0, depth);
+}
+
+int getLOG(int depth) {
+    int LOG = 0;
+    while ((1 << LOG) < depth) {
+        LOG++;
+    }
+    return LOG;
+}
+
+int goUp(int x, int k, vector<vector<int>>& anc, int LOG) {
+    for(int i = LOG - 1; i >= 0; --i) {
+        if ((1 << i) <= k) {
+            k -= (1 << i);
+            x = anc[x][i];
+        }
+    }
+    return x;
+}
+
+int main()  {
+
+    int n, q;
+    cin >> n >> q;
+    vector<int> par(n);
+    par[0] = 0;
+    for(int i = 1; i < n; ++i) {
+        int boss;
+        cin >> boss;
+        boss--;
+        par[i] = boss;
+    }
+
+    vector<int> depth(n);
+    int d = getDepth(0, par, depth);
+    int LOG = getLOG(d) + 1;
+    vector<vector<int>> anc(n, vector<int>(LOG));
+
+    for(int i = 0; i < n; ++i) {
+        anc[i][0] = par[i];
+    }
+
+    for(int j = 1; j < LOG; ++j) {
+        for(int i = 1; i < n; ++i) {
+            anc[i][j] = anc[ anc[i][j - 1] ][j - 1];
+        }
+    }
+
+    while (q--) {
+        int x, k;
+        cin >> x >> k;
+        x--;
+
+        if(k > depth[x]) {
+            cout << "-1\n";
+        }
+        else {
+            cout << goUp(x, k, anc, LOG) + 1 << "\n";
+        }
+    }
+
+    return 0;
+}