Leetcode 1361 Validate Binary Tree Nodes

All explained in the code below

class Solution {
    public boolean validateBinaryTreeNodes(
            int n, int[] leftChild, int[] rightChild) {
        // Rules for a tree
        // - each node that is not the root is exactly one time a 
        // left or right child (should be in right or left)
        // - the rooth is not in left child or right child
        // - all nodes should be accessible from the root
        // all these conditions are necessare and sufficient (it is 
        // possible to show it)
        // From all these we get the following algorithm:
        // 1. Find the root and make sure each other node appears 
        // only once the leftChild or rightChild
        // 2. bfs from the root
        // return true if the dfs visited all nodes
        Set<Integer> nodes = new HashSet<>();
        for (int i = 0; i < n; i++) {
            nodes.add(i);
        }
        for (int val : leftChild) {
            if (val != -1) {
                if (!nodes.contains(val)) {
                    // appears many times
                    return false;
                }
                nodes.remove(val);
            }
        }
        for (int val : rightChild) {
            if (val != -1) {
                if (!nodes.contains(val)) {
                    // appears many times
                    return false;
                }
                nodes.remove(val);
            }
        }
        if (nodes.size() != 1) {
            return false;
        }
        int root = nodes.iterator().next()…