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130.java
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130.java
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//https://leetcode.com/problems/surrounded-regions/
public class UF{
public int[] parent;
public int[] height;
public int count;
public UF(int m, int n){
count=m*n;
parent=new int[m*n + 1];
height=new int[m*n + 1];
for(int i=0;i<m*n;i++){
parent[i]=i;
}
}
public boolean find(int i,int j){
return findRoot(i)==findRoot(j);
}
public int findRoot(int i){
while(parent[i]!=i) i=parent[i];
return i;
}
public void union(int i, int j){
i=findRoot(i);
j=findRoot(j);
if(i==j) return;
if(height[i]>height[j]){
parent[j]=i;
}
else if(height[i]<height[j]){
parent[i]=j;
}
else{
parent[i]=j;
height[j]++;
}
count--;
}
}
public void solve(char[][] board) {
if(board==null || board.length==0 || board[0].length==0) return;
int m=board.length;
int n=board[0].length;
UF unionFind=new UF(m,n);
for(int i=0;i<m;i++){
for(int j=0;j<n;j++){
if((i==0 || i==m-1 || j==0 || j==n-1) && (board[i][j])=='O'){
unionFind.union(i*n+j, n*m);
}
else if(board[i][j]=='O'){
int p=i*n+j;
if(i<m-1 && board[i+1][j]=='O'){
unionFind.union(p,p+n);
}
if(i>0 && board[i-1][j]=='O'){
unionFind.union(p,p-n);
}
if(j<n-1 && board[i][j+1]=='O'){
unionFind.union(p, p+1);
}
if(j>0 && board[i][j-1]=='O'){
unionFind.union(p, p-1);
}
}
}
}
for(int i=0;i<m;i++){
for(int j=0;j<n;j++){
if(!unionFind.find(i*n+j, n*m)){
board[i][j]='X';
}
}
}
}