其实并查集顾名思义就是有“合并集合”和“查找集合中的元素”两种操作的关于数据结构的一种算法。
并查集是一种用来管理元素分组情况的数据结构,并查集可以高效地进行如下操作：
- 查询元素a和元素b是否属于同一组
- 合并元素a和元素b所在组
定义
其实并查集顾名思义就是有“合并集合”和“查找集合中的元素”两种操作的关于数据结构的一种算法。
并查集是一种用来管理元素分组情况的数据结构,并查集可以高效地进行如下操作：
- 查询元素a和元素b是否属于同一组
- 合并元素a和元素b所在组
并查集的结构
用数组表达的树表示
例子
set s= {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}
pairs of equivalence:(0 4),(3 1),(6 10),(8 9),(7 4),(6 8),(3 5),(2 11)，(11 0)
Initial:{0}，{1}，{2}，{3}，{4}，{5}，{6}，{7}，{8}，{9}, {10}，{11}
union(0,4): {0，4}，{1}，{2}，{3}，{5}，{6}，{7}，{8}，{9}, {10}，{11}
union(3,1): {0，4}，{1，3}，{2}，{5}，{6}，{7}，{8}，{9}，{10}, {11}
基本功能
并查集通常有两个基本操作，find与union
public class DisjSets{
public DisjSets( int numElements )
public void union( int root1, int root2 );
public int find( int x )
private int[] parent;
}
find(x)
find(7)=find(8)=find(9)=find(1)=1
find(2)=find(10)=find(5)=5
union(i, j)
一些问题和改进
如果可能出现很差的情况，增加复杂度
improve Union two rules:
-
Weight rule: if the number of nodes in tree i is less than the number in tree j, then make j the parent of i; otherwise,make i the parent of j.
-
Height rule: if the height of tree i is less than that of tree j, then make j the parent of i; otherwise,make i the parent of j.
Weight Rule
Besides the *parent* field, each node has *a boolean field root* .The root field is true iff the node is presently a root node. The parent field of each root node is used to keep a count of the total number of nodes in the tree.
```java
public DisjointSet(int size, RuleType ruleType) {
parant = new int[size + 1];
root = new boolean[size + 1];
for (int i = 1; i < size + 1; i++) {
root[i] = true;
parant[i] = 1;
}
rule = ruleType;
}
public int find(int e) {
while (!root[e])
e = parant[e];
break;
return e;
}
public void union(int i, int j) {
unionRoot(find(i), find(j));
}
private void unionRoot(int root1, int root2) {
if (parant[root1] < parant[root2]) { // root1 become subtree of root2
parant[root2] += parant[root1];
root[root1] = false;
parant[root1] = root2;
} else {
parant[root1] += parant[root2];
root[root2] = false;
parant[root2] = root1;
}
}
```
Height Rule
用一个数组来实现，根结点中放负数，而且是代表高度。
```java
public DisjointSet(int size, RuleType ruleType) {
parant = new int[size + 1];
for (int i = 1; i < size + 1; i++) {
parant[i] = -1;
}
rule = ruleType;
}
public int find(int e) {
while (parant[e] > 0)
e = parant[e];
break;
return e;
}
public void union(int i, int j) {
unionRoot(find(i), find(j));
}
private void unionRoot(int root1, int root2) {
if (parant[root1] < parant[root2]) {
// height 1 > height 2, 2 become subtree of 1
parant[root2] = root1;
} else if (parant[root1] > parant[root2]) {
parant[root1] = root2;
} else { // same height
parant[root1] = root2;
root2--;
}
}
```
完整实现代码
public class DisjointSet {
public static void main(String[] args) {
DisjointSet disjointSet = new DisjointSet(10,RuleType.WEIGHT_RULE);
disjointSet.union(1,2);
disjointSet.union(2,3);
disjointSet.union(4,5);
disjointSet.union(5,6);
disjointSet.union(6,7);
disjointSet.union(2,5);
disjointSet.print();
System.out.println(disjointSet.find(2));
}
public enum RuleType {
WEIGHT_RULE, HEIGHT_RULE
}
private RuleType rule;
private int[] parant;
private boolean[] root; // 用于WeightRule
public DisjointSet(int size) {
this(size, RuleType.HEIGHT_RULE); // 默认使用HeightRule
}
public DisjointSet(int size, RuleType ruleType) {
parant = new int[size + 1];
if (ruleType == RuleType.WEIGHT_RULE) {
root = new boolean[size + 1];
for (int i = 1; i < size + 1; i++) {
root[i] = true;
parant[i] = 1;
}
} else if (ruleType == RuleType.HEIGHT_RULE) {
for (int i = 1; i < size + 1; i++) {
parant[i] = -1;
}
}
rule = ruleType;
}
public int find(int e) {
switch (rule) {
case HEIGHT_RULE:
while (parant[e] > 0)
e = parant[e];
break;
case WEIGHT_RULE:
while (!root[e])
e = parant[e];
break;
}
return e;
}
public void union(int i, int j) {
unionRoot(find(i), find(j));
}
private void unionRoot(int root1, int root2) {
switch (rule) {
case HEIGHT_RULE:
if (parant[root1] < parant[root2]) {
// height 1 > height 2, 2 become subtree of 1
parant[root2] = root1;
} else if (parant[root1] > parant[root2]) {
parant[root1] = root2;
} else { // same height
parant[root1] = root2;
root2--;
}
break;
case WEIGHT_RULE:
if (parant[root1] < parant[root2]) {
// root1 become subtree of root2
parant[root2] += parant[root1];
root[root1] = false;
parant[root1] = root2;
} else {
parant[root1] += parant[root2];
root[root2] = false;
parant[root2] = root1;
}
break;
}
}
public void print(){
for (int i = 1; i <parant.length ; i++) {
if ((rule == RuleType.HEIGHT_RULE &¶nt[i]<0)||(rule==RuleType.WEIGHT_RULE&&root[i])){
System.out.println("|-----"+i);
print(i,1);
}
}
}
private void print(int father,int blank){
for (int i = 1; i < parant.length; i++) {
if((rule==RuleType.HEIGHT_RULE&¶nt[i]==father)||(rule==RuleType.WEIGHT_RULE&¶nt[i]==father&&(!root[i]))){
for (int j = 0; j < blank; j++) {
System.out.print(" ");
}
System.out.println("|-----"+i);
print(i,blank+1);
}
}
}
}