A tree whose parent node has and at most two children is called a binary tree, and an N-ary tree is a parent node with N children.
Since an N-ary tree has multiple child nodes, there is no in-order traversal, only pre-order traversal, post-order traversal, and level-order traversal are left.
preorder traversal
Similar to a binary tree, the order of traversal is
root node - left child node - other child nodes - right child node
The result of preorder traversal: [1,3,5,6,2,4]
public List<Integer> preorder(Node root) {
List<Integer> list=new ArrayList<Integer>();
if(root==null){
return list;
}
list.add(root.val);
for(int i=0;i<root.children.size();i++){
list.addAll(preorder(root.children.get(i)));
}
return list;
}
post-order traversal
After the child node is placed before the root node, the result of the
post : [5,6,3,2,4,1]
public List<Integer> postorder(Node root) {
ArrayList<Integer> list=new ArrayList<Integer>();
if(root==null){
return list;
}
for(int i=0;i<root.children.size();i++){
list.addAll(postorder(root.children.get(i)));
}
list.add(root.val);
return list;
}
level-order traversal
The result of the level-order traversal:
[
[1],
[3,2,4],
[5,6]
]
public List<List<Integer>> levelOrder(Node root) {
ArrayList<Integer> levelList=new ArrayList<Integer>();
ArrayList<List<Integer>> list=new ArrayList<List<Integer>>();
LinkedList<Node> stack=new LinkedList<Node>();
stack.offer(root);
while(!stack.isEmpty()){
int size=stack.size();
for(int i=0;i<size;i++){
Node node=stack.poll();
if(node==null)continue;
for(int j=0;j<node.children.size();j++){
stack.offer(node.children.get(j));
}
levelList.add(node.val);
}
if(levelList.size()>0)
list.add(levelList);
levelList=new ArrayList<Integer>();
}
return list;
}
Depth of N-ary tree
public int maxDepth(Node root) {
if(root==null){
return 0;
}
int max=0;
for(int i=0;i<root.children.size();i++){
int temp=maxDepth(root.children.get(i));
if(temp>max){
max=temp;
}
}
return max+1;
}
Since the N-ary tree has no special properties, there are not many common operations. All the code is in GitHub , and the serialization used is Json.