@@ -59,37 +59,41 @@ struct node {
5959
6060vector<node> tree;
6161vi depth (100005 ), parent(100005 ), section_ancestor(100005 );
62- ll maxdepth = 0 ;
63- // DFS to calculate depths of each node in O(n)
64- void dfs (ll u) {
62+ ll sqrtH = 0 , maxdepth = 0 ;
63+ 64+ void find_maxdepth (ll u) {
6565 ll i, len = tree[u].adj .size ();
6666 FOR (i,0 ,len) {
6767 ll v = tree[u].adj [i];
6868 depth[v] = depth[u] + 1 ;
6969 if (depth[v] > maxdepth) {
7070 maxdepth = depth[v];
7171 }
72- dfs (v);
72+ find_maxdepth (v);
7373 }
7474}
7575
76- // LCA preprocessing in O(n)
76+ // LCA preprocessing with DFS in O(n)
7777// Divide the tree in sqrt(H) sections where H = height of the tree
7878// section_ancestor[i] stores the ancestor of node i in the last level of upper section
79- void LCA_preprocessing (ll n) {
80- ll i, sqrtH = int (sqrt (maxdepth));
79+ void dfs (ll u) {
80+ ll i, v;
81+ if (depth[u] < sqrtH) {
82+ section_ancestor[u] = 1 ;
83+ }
84+ else if (depth[u] % sqrtH == 0 ) {
85+ section_ancestor[u] = parent[u];
86+ }
87+ else {
88+ section_ancestor[u] = section_ancestor[parent[u]];
89+ }
8190
82- FOR (i,1 ,n+1 ) {
83- if (i < sqrtH) {
84- section_ancestor[i] = 1 ;
85- }
86- else if (depth[i] % sqrtH == 0 ) {
87- section_ancestor[i] = parent[i];
88- }
89- else {
90- section_ancestor[i] = section_ancestor[parent[i]];
91- }
91+ ll len = tree[u].adj .size ();
92+ FOR (i,0 ,len) {
93+ v = tree[u].adj [i];
94+ dfs (v);
9295 }
96+ 9397}
9498
9599// Returns LCA of x and y in O(sqrt(n))
@@ -127,15 +131,14 @@ int main(int argc, char *argv[]) {
127131 tree[j].adj .pb (i);
128132 parent[i] = j;
129133 }
134+ find_maxdepth (1 );
135+ sqrtH = int (sqrt (maxdepth));
130136 dfs (1 );
131- // cout << "DFS complete ----> maxdepth = " << maxdepth << endl;
132- LCA_preprocessing (n);
133- // cout << "LCA preprocessing done" << endl;
134- // FOR(i,1,n+1) {
135- // cout << "section_ancestor[" << i <<"] = " << section_ancestor[i] << endl;
136- // }
137+ FOR (i,1 ,n+1 ) {
138+ cout << " section_ancestor[" " ] = "
139+ }
137140 x = read_ll ();
138141 y = read_ll ();
139- // cout << "LCA of " << x << " and " << y << " = " << getLCA(x,y) << endl;
142+ cout << " LCA of " " and " " = " getLCA (x,y) << endl;
140143 return 0 ;
141144}
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