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LargestCommonSubtrees.java
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package Algorithms.tree;
import java.util.*;
public class LargestCommonSubtrees {
/*
* Function: Find all the common subtrees root that have most number
*/
private static class TreeNode {
int val;
ArrayList<TreeNode> subNodes;
public TreeNode(int val, ArrayList<TreeNode> subNodes) {
this.val = val;
this.subNodes = subNodes;
}
}
public static void main(String[] args) {
/*
* 1
*
* / \ \
* 2 3 4
* / \ / \
* 5 6 8 9
* \ \
* 7 10
* */
TreeNode r1 = new TreeNode(1, new ArrayList<TreeNode>());
TreeNode r2 = new TreeNode(2, new ArrayList<TreeNode>());
TreeNode r3 = new TreeNode(3, new ArrayList<TreeNode>());
TreeNode r4 = new TreeNode(4, new ArrayList<TreeNode>());
TreeNode r5 = new TreeNode(5, new ArrayList<TreeNode>());
TreeNode r6 = new TreeNode(6, new ArrayList<TreeNode>());
TreeNode r7 = new TreeNode(7, new ArrayList<TreeNode>());
TreeNode r8 = new TreeNode(8, new ArrayList<TreeNode>());
TreeNode r9 = new TreeNode(9, new ArrayList<TreeNode>());
TreeNode r10 = new TreeNode(10, new ArrayList<TreeNode>());
r1.subNodes.add(r2);
r1.subNodes.add(r3);
r1.subNodes.add(r4);
r2.subNodes.add(r5);
r2.subNodes.add(r6);
r6.subNodes.add(r7);
r4.subNodes.add(r8);
r4.subNodes.add(r9);
r9.subNodes.add(r10);
ArrayList<ArrayList<TreeNode>> ret = largestCommonSubtrees(r1);
for (ArrayList<TreeNode> arrayl: ret) {
for (TreeNode t: arrayl) {
System.out.println(t.val);
}
}
}
public static ArrayList<ArrayList<TreeNode>> largestCommonSubtrees(TreeNode root) {
if (root == null) {
return null;
}
// store all the tree nodes to a arrayList.
ArrayList<TreeNode> nodes = new ArrayList<TreeNode>();
traversalTree(root, nodes);
int maxNum = 0;
HashMap<Integer, Integer> hash = new HashMap<Integer, Integer>();
TreeNode r1 = null;
TreeNode r2 = null;
// compare all the nodes.
int size = nodes.size();
for (int i = 0; i < size; i++) {
for (int j = 0; j < size; j++) {
if (i == j) {
continue;
}
int num = compareTree(nodes.get(i), nodes.get(j), hash);
if (num > maxNum) {
maxNum = num;
r1 = nodes.get(i);
r2 = nodes.get(j);
}
}
}
ArrayList<ArrayList<TreeNode>> retNew = new ArrayList<ArrayList<TreeNode>>();
retNew.add(new ArrayList<TreeNode>());
retNew.get(0).add(r1);
retNew.get(0).add(r2);
return retNew;
}
// compare two tree, if same, return the number of leafs. if no, return -1;
public static int compareTree(TreeNode r1, TreeNode r2, HashMap<Integer, Integer> hash) {
if (r1 == null && r2 == null) {
return 0;
}
if (r1 == null || r2 == null) {
return -1;
}
// the number of subtrees should be same.
if (r1.subNodes.size() != r2.subNodes.size()) {
return -1;
}
int num = 1; // the root itself.
for (int i = 0; i < r1.subNodes.size(); i++) {
// get the subNode of r1.
TreeNode subNode1 = r1.subNodes.get(i);
TreeNode subNode2 = r2.subNodes.get(i);
int HashCode = hashCode(subNode1, subNode2);
Integer numNode = hash.get(HashCode);
if (numNode == null) {
numNode = compareTree(subNode1, subNode2, hash);
}
if (numNode == -1) {
// not the same, should return;
num = -1;
break;
} else {
num += numNode;
continue;
}
}
int hashCodeRoot = hashCode(r1, r2);
hash.put(hashCodeRoot, num);
return num;
}
public static int hashCode(TreeNode r1, TreeNode r2) {
int hash = r1.hashCode() * 31 + r2.hashCode();
return hash;
}
public static void traversalTree(TreeNode root, ArrayList<TreeNode> ret) {
if (root == null) {
return;
}
ret.add(root);
// add all the sub nodes.
if (root.subNodes != null) {
for(TreeNode t: root.subNodes) {
traversalTree(t, ret);
}
}
}
}