题目:

给定一个 n 叉树的根节点  root ,返回 其节点值的 前序遍历 。

n 叉树 在输入中按层序遍历进行序列化表示,每组子节点由空值 null 分隔(请参见示例)。

示例 1:

输入:root = [1,null,3,2,4,null,5,6]

输出:[1,3,5,6,2,4]

示例 2:

输入:root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]

输出:[1,2,3,6,7,11,14,4,8,12,5,9,13,10]

代码实现:

class Solution {
public List<Integer> preorder(Node root) {
List<Integer> res = new ArrayList<Integer>();
if (root == null) {
return res;
}
Map<Node, Integer> map = new HashMap<Node, Integer>();
Deque<Node> stack = new ArrayDeque<Node>();
Node node = root;
while (!stack.isEmpty() || node != null) {
while (node != null) {
res.add(node.val);
stack.push(node);
List<Node> children = node.children;
if (children != null && children.size() > 0) {
map.put(node, 0);
node = children.get(0);
} else {
node = null;
}
}
node = stack.peek();
int index = map.getOrDefault(node, -1) + 1;
List<Node> children = node.children;
if (children != null && children.size() > index) {
map.put(node, index);
node = children.get(index);
} else {
stack.pop();
map.remove(node);
node = null;
}
}
return res;
}
}