g0801_0900.s0827_making_a_large_island.Solution Maven / Gradle / Ivy
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Java-based LeetCode algorithm problem solutions, regularly updated
package g0801_0900.s0827_making_a_large_island;
// #Hard #Array #Depth_First_Search #Breadth_First_Search #Matrix #Union_Find
import java.util.HashMap;
public class Solution {
private int[] p;
private int[] s;
private void makeSet(int x, int y, int rl) {
int a = x * rl + y;
p[a] = a;
s[a] = 1;
}
private void comb(int x1, int y1, int x2, int y2, int rl) {
int a = find(x1 * rl + y1);
int b = find(x2 * rl + y2);
if (a == b) {
return;
}
if (s[a] < s[b]) {
int t = a;
a = b;
b = t;
}
p[b] = a;
s[a] += s[b];
}
private int find(int a) {
if (p[a] == a) {
return a;
}
p[a] = find(p[a]);
return p[a];
}
public int largestIsland(int[][] grid) {
int rl = grid.length;
int cl = grid[0].length;
p = new int[rl * cl];
s = new int[rl * cl];
for (int i = 0; i < rl; i++) {
for (int j = 0; j < cl; j++) {
if (grid[i][j] == 0) {
continue;
}
makeSet(i, j, rl);
if (i > 0 && grid[i - 1][j] == 1) {
comb(i, j, i - 1, j, rl);
}
if (j > 0 && grid[i][j - 1] == 1) {
comb(i, j, i, j - 1, rl);
}
}
}
int m = 0;
int t;
HashMap sz = new HashMap<>();
for (int i = 0; i < rl; i++) {
for (int j = 0; j < cl; j++) {
if (grid[i][j] == 0) {
// find root, check if same and combine size
t = 1;
if (i > 0 && grid[i - 1][j] == 1) {
sz.put(find((i - 1) * rl + j), s[find((i - 1) * rl + j)]);
}
if (j > 0 && grid[i][j - 1] == 1) {
sz.put(find(i * rl + j - 1), s[find(i * rl + j - 1)]);
}
if ((i < rl - 1) && grid[i + 1][j] == 1) {
sz.put(find((i + 1) * rl + j), s[find((i + 1) * rl + j)]);
}
if ((j < cl - 1) && grid[i][j + 1] == 1) {
sz.put(find(i * rl + j + 1), s[find(i * rl + j + 1)]);
}
for (int val : sz.values()) {
t += val;
}
m = Math.max(m, t);
sz.clear();
} else {
m = Math.max(m, s[i * rl + j]);
}
}
}
return m;
}
}