Number of Islands II

You are given an empty m x n grid initially filled with water. We may perform an addLand operation where positions[i] = [ri, ci] turns the cell (ri, ci) into land. Examples Example 1: Input: m = 3, n = 3, positions = [[0,0],[0,1],[1,2],[2,1],[1,1]] Output: [1,1,2,3,1] Explanation: The last addition connects three previously separate islands into one. Example 2: Input: m = 1, n = 1, positions = [[0,0]] Output: [1] Constraints 1 <= m, n <= 10^4 1 <= m * n <= 10^4 0 <= positions.length <= 10^4 0 <= ri < m 0 <= ci < n