Question Definition
Given an integer n, generate all structurally unique BST’s (binary search trees) that store values 1…n.
For example, Given n = 3, your program should return all 5 unique BST’s shown below.
1 3 3 2 1
\ / / / \ \
3 2 1 1 3 2
/ / \ \
2 1 2 3
Java Solution
public List<TreeNode> generateTrees(int n) {
if(n == 0)
return new LinkedList<>();
return genTrees(1,n);
}
public List<TreeNode> genTrees (int start, int end)
{
List<TreeNode> list = new ArrayList<TreeNode>();
if(start>end)
{
list.add(null);
return list;
}
if(start == end){
list.add(new TreeNode(start));
return list;
}
List<TreeNode> left,right;
for(int i=start;i<=end;i++)
{
left = genTrees(start, i-1);
right = genTrees(i+1,end);
for(TreeNode lnode: left)
{
for(TreeNode rnode: right)
{
TreeNode root = new TreeNode(i);
root.left = lnode;
root.right = rnode;
list.add(root);
}
}
}
return list;
}
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