Daily Coding Problem: Problem #793

/**
 * This problem was asked by Yahoo.
Recall that a full binary tree is one in which each node is either a leaf node, or has two children.
Given a binary tree, convert it to a full one by removing nodes with only one child.

For example, given the following tree:
     0
    / \
   1   2
  /     \
 3       4
  \     / \
   5   6   7

You should convert it to:
    0
   / \
  5   4
     / \
    6   7
 * */
class Problem_793 {
    /*
    * solution: post-order:left->right->root, to check each node's children if null.
    * we form the new tree bottom up, starting from the leaves towards the root.
    * By the time we process the current node, both its left and right subtrees were already processed.
    * */
    fun covertTreeToFull(node: Node?): Node? {
        if (node == null) {
            return null
        }
        node.left = covertTreeToFull(node.left)
        node.right = covertTreeToFull(node.right)
        if (node.left == null && node.right == null) {
            return node
        }
        if (node.left == null) {
            return node.right
        }
        if (node.right == null) {
            return node.left
        }
        return node
    }

    class Node(value: Int) {
        var left: Node? = null
        var right: Node? = null
    }
}