N叉树的前后序迭代遍历

1、前序

//Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val,List<Node>; _children) {
        val = _val;
        children = _children;
    }
};
class Solution {
    public List<Integer> postorder(Node root) {
        Stack<Node> s = new Stack<>();
        List<Integer> result = new LinkedList<>();
        Node x;
        if(root!=null){
            s.push(root);
        }else{
            return result;
        }
        while(!s.isEmpty()){
            x = s.pop();
            result.add(x.val);
            for(int i = x.children.size()-1;i>-1;i++){
                s.push(x.children.get(i));
            }
        }
        return result;
    }
}

2、后序

// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val,List<Node> _children) {
        val = _val;
        children = _children;
    }
};
class Solution {
    public List<Integer> postorder(Node root) {
        Stack<Node> s = new Stack<>();
        List<Integer> result = new LinkedList<>();
        Node x;
        if(root!=null){
            s.push(root);
        }else{
            return result;
        }
        while(!s.isEmpty()){
            x = s.pop();
            for(Node n:x.children){
                s.push(n);
            }
            result.add(0,x.val);
        }
        return result;
    }
}

额外:二叉树的中序遍历

class solution{
        private List<Integer> result = new LinkedList<Integer>();
        public List<Integer> inorderTraversal(TreeNode root) {
        if(root == null)
            return result;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        do {
        //依次将左节点均加入栈中
            while(root != null) {
            stack.push(root);
            root = root.left;
            }
            if (!stack.isEmpty()) {
            root = stack.pop();
            result.add(root.val);
            //这一步若右节点为null则用于躲避上面的while循环
            //若不为null则会将右节点上面的循环入栈
            //因此出栈顺序是最深处的左中右(右无子树,若有还要延迟该层右节点的出栈)
            root = root.right;
            }
        } while(!stack.isEmpty() || root != null);
        return result;
    }
}

总结：

由于N叉树没有左子树右子树这样说法，所以没用中序遍历方式，这三种方式用递归较为简单，而迭代由于其输出的不同需要要用到堆栈这一特性，即FILO，但是在树的层次遍历中，由于是父亲节点优先于子节点，既遵循FIFO，故采用队列。