building tree view from given child-parent pairs in hashmap

Ex: The child-parent pairs are in a hashmap 'childParentMap' as shown below. The root node is with value 0.
1 -> 0
2 -> 1
3 -> 2
4 -> 2
5 -> 0

The tree node data structure has an int value and list of children tree nodes.

class TreeNode:
    int value
    IList children
   
    //constructor
    TreeNode(int value):
        this.value = value
        children = new List()


The code first traverses through the childParentMap hashmap to build the value->TreeNode hashmap 'valueNodeMap'. It traverses through it again to get child and parent values, their equivalent nodes and adds the child node to the parent node. The root node with value 0 is created separately (root of the tree).

TreeNode createTreeView(Hashmap childParentMap):
    // build value->TreeNode map
    Hashmap valueNodeMap = new Hashmap()
    foreach (KeyValuePair kvPair in childParentMap):
        int value = kvPair.Key
        TreeNode node = new TreeNode(value)
        if !valueNodeMap.ContainsKey(value):
            valueNodeMap.Add(value, node)
   
    // add root value 0 and root node to valueNodeMap
    int rootValue = 0;
    TreeNode rootNode = new TreeNode(rootValue);
    valueNodeMap.Add(rootValue, rootNode);
   
    // for child and parent values, find equivalent nodes and
    // add child node as child to parent node
    foreach (KeyValuePair kvPair in childParentMap):
        int childValue = kvPair.Key
        int parentValue = kvPair.Value
        TreeNode childNode = valueNodeMap[childValue]
        TreeNode parentNode = valueNodeMap[parentValue]
       
        parentNode.children.Add(childNode)
   
    // return root
    return rootNode


Time is O(n).

There is a recursive solution too but it takes O(n^2) time.

The code is similar to building a graph from an adjacency list.

[H/T ai, as, sc, jf, se]