1+ class Solution {
2+ public List <Integer > numIslands2 (int m , int n , int [][] positions ) {
3+ int x [] = { -1 , 1 , 0 , 0 };
4+ int y [] = { 0 , 0 , -1 , 1 };
5+ 6+ UnionFind dsu = new UnionFind (m * n );
7+ 8+ List <Integer > answer = new ArrayList <>();
9+ 10+ for (int [] position : positions ) {
11+ 12+ int landPosition = position [0 ] * n + position [1 ];
13+ 14+ dsu .addLand (landPosition );
15+ 16+ for (int i = 0 ; i < 4 ; i ++) {
17+ 18+ int neighborX = position [0 ] + x [i ];
19+ int neighborY = position [1 ] + y [i ];
20+ 21+ int neighborPosition = neighborX * n + neighborY ;
22+ 23+ if (neighborX >= 0 && neighborX < m && neighborY >= 0 && neighborY < n && dsu .isLand (neighborPosition )) {
24+ 25+ dsu .union (landPosition , neighborPosition );
26+ 27+ }
28+ }
29+ 30+ answer .add (dsu .numberOfIslands ());
31+ 32+ }
33+ 34+ return answer ;
35+ }
36+ }
37+ 38+ class UnionFind {
39+ 40+ int [] parent ;
41+ int [] rank ;
42+ 43+ int count ;
44+ 45+ public UnionFind (int size ) {
46+ 47+ parent = new int [size ];
48+ rank = new int [size ];
49+ 50+ for (int i = 0 ; i < size ; i ++) {
51+ 52+ parent [i ] = -1 ;
53+ 54+ }
55+ 56+ count = 0 ;
57+ }
58+ 59+ public void addLand (int x ) {
60+ 61+ if (parent [x ] >= 0 ) {
62+ return ;
63+ }
64+ 65+ parent [x ] = x ;
66+ count ++;
67+ 68+ }
69+ 70+ public boolean isLand (int x ) {
71+ 72+ if (parent [x ] >= 0 ) {
73+ return true ;
74+ } else {
75+ return false ;
76+ }
77+ 78+ }
79+ 80+ int numberOfIslands () {
81+ return count ;
82+ }
83+ 84+ public int find (int x ) {
85+ 86+ if (parent [x ] != x ) {
87+ parent [x ] = find (parent [x ]);
88+ }
89+ 90+ return parent [x ];
91+ 92+ }
93+ 94+ public void union (int x , int y ) {
95+ 96+ int xset = find (x );
97+ int yset = find (y );
98+ 99+ if (xset == yset ) {
100+ return ;
101+ } else if (rank [xset ] < rank [yset ]) {
102+ parent [xset ] = yset ;
103+ } else if (rank [xset ] > rank [yset ]) {
104+ parent [yset ] = xset ;
105+ } else {
106+ parent [yset ] = xset ;
107+ rank [xset ]++;
108+ }
109+ 110+ count --;
111+ 112+ }
113+ }
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