Solution.go

type unionFind struct {
	p, size []int
}

func newUnionFind(n int) *unionFind {
	p := make([]int, n)
	size := make([]int, n)
	for i := range p {
		p[i] = i
		size[i] = 1
	}
	return &unionFind{p, size}
}

func (uf *unionFind) find(x int) int {
	if uf.p[x] != x {
		uf.p[x] = uf.find(uf.p[x])
	}
	return uf.p[x]
}

func (uf *unionFind) union(a, b int) bool {
	pa, pb := uf.find(a), uf.find(b)
	if pa == pb {
		return false
	}
	if uf.size[pa] > uf.size[pb] {
		uf.p[pb] = pa
		uf.size[pa] += uf.size[pb]
	} else {
		uf.p[pa] = pb
		uf.size[pb] += uf.size[pa]
	}
	return true
}

func numIslands2(m int, n int, positions [][]int) (ans []int) {
	uf := newUnionFind(m * n)
	grid := make([][]int, m)
	for i := range grid {
		grid[i] = make([]int, n)
	}
	dirs := [5]int{-1, 0, 1, 0, -1}
	cnt := 0
	for _, p := range positions {
		i, j := p[0], p[1]
		if grid[i][j] == 1 {
			ans = append(ans, cnt)
			continue
		}
		grid[i][j] = 1
		cnt++
		for k := 0; k < 4; k++ {
			x, y := i+dirs[k], j+dirs[k+1]
			if x >= 0 && x < m && y >= 0 && y < n && grid[x][y] == 1 && uf.union(i*n+j, x*n+y) {
				cnt--
			}
		}
		ans = append(ans, cnt)
	}
	return
}