183132I32II32III24

Posted 2022-12-12 保护眼睛

剑指offer

• • • 剑指 Offer 18. 删除链表的节点
    • 剑指 Offer 24. 反转链表
    • 剑指 Offer 31. 栈的压入、弹出序列
    • 剑指 Offer 32 - I. 从上到下打印二叉树
    • 剑指 Offer 32 - II. 从上到下打印二叉树
    • 剑指 Offer 32 - III. 从上到下打印二叉树

剑指 Offer 18. 删除链表的节点

    public ListNode deleteNode(ListNode head, int val) 
        if (head == null)
            return null;
        ListNode cur = head;
        ListNode pre = new ListNode(-1);
        if (val != head.val) 
            pre.next = head;
         else 
            pre.next = head.next;
            return pre.next;
        
        while (cur != null) 
            if (cur.val == val) 
                pre.next = cur.next;
                break;
             else 
                pre = cur;
                cur = cur.next;
            
        

        return head;
    

    public ListNode deleteNode2(ListNode head, int val) 
        if (head == null) return null;
        if (head.val == val) 
            return head.next;
         else 
            head.next = deleteNode2(head.next, val);
        
        return head;
    

剑指 Offer 24. 反转链表

    public ListNode swapPairs(ListNode head) 
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode cur = dummy;

        while (cur.next != null && cur.next.next != null) 
            ListNode node1 = cur.next;
            ListNode node2 = cur.next.next;

            cur.next = node2;
            node1.next = node2.next;
            node2.next = node1;
            cur = node1;
        

        return dummy.next;
    

剑指 Offer 31. 栈的压入、弹出序列

    public boolean validateStackSequences(int[] pushed, int[] popped) 
        Stack<Integer> stack = new Stack<>();
        int i = 0;
        for (int val : pushed) 
            stack.push(val);
            while (!stack.isEmpty() && stack.peek() == popped[i]) 
                stack.pop();
                i++;
            
        
        return stack.isEmpty();
    

剑指 Offer 32 - I. 从上到下打印二叉树

    public int[] levelOrder(TreeNode root) 
        List<Integer> res = new ArrayList<>();
        if (root == null)
            return tran(res);

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) 
            TreeNode cur = queue.poll();
            res.add(cur.val);
            if (cur.left != null) 
                queue.offer(cur.left);
            

            if (cur.right != null) 
                queue.offer(cur.right);
            
        

        return tran(res);
    

    public int[] tran(List<Integer> list) 
        int size = list.size();
        int[] res = new int[size];
        for (int i = 0; i < size; i++) 
            res[i] = list.get(i);
        

        return res;
    

剑指 Offer 32 - II. 从上到下打印二叉树

    public List<List<Integer>> levelOrder(TreeNode root) 
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) 
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            while (size != 0) 
                TreeNode cur = queue.poll();
                list.add(cur.val);
                if (cur.left != null) 
                    queue.offer(cur.left);
                
                if (cur.right != null) 
                    queue.offer(cur.right);
                
                size--;
            
            res.add(list);
        
        return res;
    

剑指 Offer 32 - III. 从上到下打印二叉树

    boolean flag = true;

    public List<List<Integer>> levelOrder(TreeNode root) 
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) return res;
        Queue<TreeNode> queue = new LinkedList<>();

        queue.offer(root);
        while (!queue.isEmpty()) 
            int size = queue.size();
            List<Integer> list = new ArrayList<>();

            for (int i = 0; i < size; i++) 
                TreeNode cur = queue.poll();

                list.add(cur.val);

                if (cur.left != null) 
                    queue.offer(cur.left);
                

                if (cur.right != null) 
                    queue.offer(cur.right);
                
            
            if (flag) 
                res.add(list);
                flag = false;
             else 
                Collections.reverse(list);
                res.add(list);
                flag = true;
            
        

        return res;
    


    public List<List<Integer>> levelOrder(TreeNode root) 
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) 
            int size = queue.size();
            LinkedList<Integer> tmpList = new LinkedList<>();

            for (int i = 0; i < size; i++) 
                TreeNode cur = queue.poll();
                if (res.size() % 2 != 0) 
                    tmpList.addFirst(cur.val);
                 else 
                    tmpList.addLast(cur.val);
                
                if (cur.left != null) 
                    queue.offer(cur.left);
                

                if (cur.right != null) 
                    queue.offer(cur.right);
                
            
            res.add(tmpList);
        

        return res;