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LeastCommonAncestor.java
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LeastCommonAncestor.java
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import java.util.ArrayList;
public class LeastCommonAncestor {
public static int [][] jump= new int[19][100010];
public static int [] d= new int[100010];
public static int [] par= new int[100010];
boolean [] vis = new boolean[100010];
public static ArrayList<Integer>[] adj;
public static int n;
public static void main(String[] args) {
par[0]=-1;
go(0,-1);
for (int i = 0; i < n; i++) {
jump[0][i]= par[i];
}
for (int i = 1; i < 19; i++) {
for (int j = 0; j < n; j++) {
if(jump[i-1][j]==-1) jump[i][j]= -1;
else jump[i][j]= jump[i-1][jump[i-1][j]];
}
}
}
public static int lca(int a, int b) {
//a is the higher one in the tree
if(d[a]<d[b]) {
int temp= a;
a=b;
b=temp;
}
for(int i=18; i>=0; i--) {
int to= jump[i][a];
if(to!=-1 && d[to]>=d[b]) {
a= to;
}
}
if(a==b) return a;
for(int i=18; i>=0; i--) {
int toa= jump[i][a];
int tob= jump[i][b];
if(toa==-1||tob==-1) continue;
if(toa!=tob) {
a=toa;
b=tob;
}
}
return jump[0][a];
}
public static void go(int on, int p) {
for(int nex: adj[on]) {
if(p==nex) continue;
par[nex]= on;
d[nex]= d[par[nex]]+1;
go(nex, on);
}
}
}