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Adding a data structure to store the intermediate sums #1

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Adding a data structure to store the intermediate sums #1

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35 changes: 26 additions & 9 deletions java/src/binarytrees/BstCountTrees.java
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Original file line number Diff line number Diff line change
Expand Up @@ -14,18 +14,35 @@
* For example, countTrees(4) should return 14, since there
* are 14 structurally unique binary search trees that store 1, 2, 3, and 4.
*/
public class BstCountTrees {

public static <T> long countTrees(int numKeys) {
if (numKeys <= 0) {
return 0;
}

public class BstCountTrees
{

private static <T> long countTreesInternal(int numKeys, long[] savedSums)
{

long sum = 0L;
for (int root = 1; root <= numKeys; root++) {
sum += countTrees(root - 1) * countTrees(numKeys - root);
for (int root = 1; root <= numKeys; root++)
{
if (savedSums[root - 1] == 0)
savedSums[root - 1] = countTreesInternal(root - 1,savedSums);
if(savedSums[numKeys - root] == 0)
savedSums[numKeys - root] = countTreesInternal(numKeys - root,savedSums);
sum += savedSums[root - 1] * savedSums[numKeys - root];
}

return sum;
}

public static <T> long countTrees(int numKeys){
long[] savedSums = new long[numKeys + 1];
savedSums[0] = 1;
return countTreesInternal(numKeys, savedSums);
}
public static void main(String[] args)
{
long numTrees = BstCountTrees.countTrees(10);
System.out.println(numTrees);

}

}