modify code on class

src/class09_15/Code01_FriendCircles.java

@@ -7,10 +7,11 @@ public class Code01_FriendCircles {
 
 	public static int findCircleNum(int[][] M) {
 		int N = M.length;
+		// {0} {1} {2} {N-1}
 		UnionFind unionFind = new UnionFind(N);
 		for (int i = 0; i < N; i++) {
 			for (int j = i + 1; j < N; j++) {
-				if (M[i][j] == 1) {
+				if (M[i][j] == 1) { // i和j互相认识
 					unionFind.union(i, j);
 				}
 			}
@@ -19,9 +20,14 @@ public class Code01_FriendCircles {
 	}
 
 	public static class UnionFind {
+		// parent[i] = k ： i的父亲是k
 		private int[] parent;
+		// size[i] = k ： 如果i是代表节点，size[i]才有意义，否则无意义
+		// i所在的集合大小是多少
 		private int[] size;
+		// 辅助结构
 		private int[] help;
+		// 一共有多少个集合
 		private int sets;
 
 		public UnionFind(int N) {
@@ -35,6 +41,8 @@ public class Code01_FriendCircles {
 			}
 		}
 
+		// 从i开始一直往上，往上到不能再往上，代表节点，返回
+		// 这个过程要做路径压缩
 		private int find(int i) {
 			int hi = 0;
 			while (i != parent[i]) {

src/class09_15/Code02_NumberOfIslands.java

@@ -23,6 +23,7 @@ public class Code02_NumberOfIslands {
 		return islands;
 	}
 
+	// 从(i,j)这个位置出发，把所有练成一片的'1'字符，变成0
 	public static void infect(char[][] board, int i, int j) {
 		if (i < 0 || i == board.length || j < 0 || j == board[0].length || board[i][j] != '1') {
 			return;
@@ -49,6 +50,7 @@ public class Code02_NumberOfIslands {
 		}
 		UnionFind1<Dot> uf = new UnionFind1<>(dotList);
 		for (int j = 1; j < col; j++) {
+			// (0,j)  (0,0)跳过了  (0,1) (0,2) (0,3)
 			if (board[0][j - 1] == '1' && board[0][j] == '1') {
 				uf.union(dots[0][j - 1], dots[0][j]);
 			}
@@ -192,10 +194,12 @@ public class Code02_NumberOfIslands {
 			}
 		}
 
+		// (r,c) -> i
 		private int index(int r, int c) {
 			return r * col + c;
 		}
 
+		// 原始位置 -> 下标
 		private int find(int i) {
 			int hi = 0;
 			while (i != parent[i]) {

src/class09_15/Code03_NumberOfIslandsII.java

@@ -1,6 +1,7 @@
 package class09_15;
 
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
 
 // 本题为leetcode原题
@@ -8,8 +9,8 @@ import java.util.List;
 // 所有方法都可以直接通过
 public class Code03_NumberOfIslandsII {
 
-	public static List<Integer> numIslands2(int m, int n, int[][] positions) {
+	public static List<Integer> numIslands21(int m, int n, int[][] positions) {
-		UnionFind uf = new UnionFind(m, n);
+		UnionFind1 uf = new UnionFind1(m, n);
 		List<Integer> ans = new ArrayList<>();
 		for (int[] position : positions) {
 			ans.add(uf.connect(position[0], position[1]));
@@ -17,7 +18,7 @@ public class Code03_NumberOfIslandsII {
 		return ans;
 	}
 
-	public static class UnionFind {
+	public static class UnionFind1 {
 		private int[] parent;
 		private int[] size;
 		private int[] help;
@@ -25,7 +26,7 @@ public class Code03_NumberOfIslandsII {
 		private final int col;
 		private int sets;
 
-		public UnionFind(int m, int n) {
+		public UnionFind1(int m, int n) {
 			row = m;
 			col = n;
 			sets = 0;
@@ -74,16 +75,87 @@ public class Code03_NumberOfIslandsII {
 			}
 		}
 
-		public int connect(int i, int j) {
+		public int connect(int r, int c) {
-			int index = index(i, j);
+			int index = index(r, c);
 			if (size[index] == 0) {
 				parent[index] = index;
 				size[index] = 1;
 				sets++;
-				union(i - 1, j, i, j);
+				union(r - 1, c, r, c);
-				union(i + 1, j, i, j);
+				union(r + 1, c, r, c);
-				union(i, j - 1, i, j);
+				union(r, c - 1, r, c);
-				union(i, j + 1, i, j);
+				union(r, c + 1, r, c);
+			}
+			return sets;
+		}
+
+	}
+
+	// 课上讲的如果m*n比较大，会经历很重的初始化，而k比较小，怎么优化的方法
+	public static List<Integer> numIslands22(int m, int n, int[][] positions) {
+		UnionFind2 uf = new UnionFind2();
+		List<Integer> ans = new ArrayList<>();
+		for (int[] position : positions) {
+			ans.add(uf.connect(position[0], position[1]));
+		}
+		return ans;
+	}
+
+	public static class UnionFind2 {
+		private HashMap<String, String> parent;
+		private HashMap<String, Integer> size;
+		private ArrayList<String> help;
+		private int sets;
+
+		public UnionFind2() {
+			parent = new HashMap<>();
+			size = new HashMap<>();
+			help = new ArrayList<>();
+			sets = 0;
+		}
+
+		private String find(String cur) {
+			while (!cur.equals(parent.get(cur))) {
+				help.add(cur);
+				cur = parent.get(cur);
+			}
+			for (String str : help) {
+				parent.put(str, cur);
+			}
+			help.clear();
+			return cur;
+		}
+
+		private void union(String s1, String s2) {
+			if (parent.containsKey(s1) && parent.containsKey(s2)) {
+				String f1 = find(s1);
+				String f2 = find(s2);
+				if (!f1.equals(f2)) {
+					int size1 = size.get(f1);
+					int size2 = size.get(f2);
+					String big = size1 >= size2 ? f1 : f2;
+					String small = big == f1 ? f2 : f1;
+					parent.put(small, big);
+					size.put(big, size1 + size2);
+					sets--;
+				}
+			}
+		}
+
+		public int connect(int r, int c) {
+			String key = String.valueOf(r) + "_" + String.valueOf(c);
+			if (!parent.containsKey(key)) {
+				parent.put(key, key);
+				size.put(key, 1);
+				sets++;
+				String up = String.valueOf(r - 1) + "_" + String.valueOf(c);
+				String down = String.valueOf(r + 1) + "_" + String.valueOf(c);
+				String left = String.valueOf(r) + "_" + String.valueOf(c - 1);
+				String right = String.valueOf(r) + "_" + String.valueOf(c + 1);
+				union(up, key);
+				union(down, key);
+				union(left, key);
+				union(right, key);
 			}
 			return sets;
 		}