玩轉數據結構<四> 鏈表與遞歸

根據LeetCode初步瞭解遞歸算法

• 代碼實現

//listNode
public class ListNode {
        int val;
        ListNode next;
        ListNode(int x) { val = x; }
        // 鏈表節點的構造函數
        //使用arr爲參數，建立一個鏈表，當前的ListNode爲鏈表頭結點
        publc ListBide(int[] arr){
            if(arr == null || arr.length == 0 ){
                throw new IllegalArgumentException("arr can not be empty");
                this.val = arr[0];
                ListNode cur = this;
                for(int i = 1;i < arr.length; i++){
                    cur.next = new ListNode(arr[i]);
                    cur = cur.next;
                }
            }
        }
        
        // 以當前節點爲頭結點的鏈表信息字符串
        @Override
        public String toString(){
            StringBuilder res = new StringBuilder();
            ListNode cur = this;
            while(cur != null){
                res.append(cur.val + "->");
                cur = cur.next;
            }
            res.append("NULL");
            return res.toString();
        }
}

//主代碼
public class Solution {

    public ListNode removeElements(ListNode head,int val){
        while(head != null && head.val == val){
            ListNode delNode = head;
            head = head.next;
            delNode.next = null;
        }
        if(head == null){
            return null;
        }
        ListNode prev = head;
        while(prev.next != null){
            if(prev.next.val == val){
                ListNode delNode = prev.next;
                prev.next = delNode.next;
                delNode.next = null;
            }else{
                prev = prev.next;
            }
        }
        return head;
    }
}

//虛擬頭結點寫法
public class Solution {

    public ListNode removeElements(ListNode head,int val){
      
        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;
        
        ListNode prev = dummyHead;
        while(prev.next != null){
            if(prev.next.val == val){
                ListNode delNode = prev.next;
                prev.next = delNode.next;
                delNode.next = null;
            }else{
                prev = prev.next;
            }
        }
        return dummyHead.next;
    }
}

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遞歸

• 示例代碼

public class Sum{
    public static int sum(int[] arr){
        return sum(arr,0);
    }
    
    //計算arr[1...n)這個區間內全部數字的和]
    private static int sum(int[] arr,int l){
        if(l == arr.length){
            return 0;
        }
        return arrp[l] + sum(arr,l + 1);
    }
    public static void main(String[] args){
        int[] nums = {1,2,3,4,5,6,7,8};
        System.out.println(sum(nums));
    }
}
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鏈表和遞歸

• 代碼示例

class Solution3{
    public ListNode removeElements(ListNode head,int val){
        if(head == null){
            return null;
        }
        ListNode res = removeElements(head.next,val);
        if(head.val == val){
            return res;
        }else{
            head.next = res;
            return head;
        }
           <!-- if(head == null){
            return null;
        }
        head.next = removeElements(head.next,val);
        return head.val == val ? head.next : head;
        -->
    }
    public static void main(String[] args){
        int[] nums = {1,2,6,3,4,5,6};
        ListNode head = new ListNode(nums);
        System.out.println(head);
        
        ListNode res = (new Solution3()).removeElements(head,6);
        System.out.println(res);
    }
}
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遞歸函數的「微觀」解讀

• 跟棧相似 只不過調用其餘函數改成了調用自身

• 代碼示例

//主代碼
public class Solution {

    public ListNode removeElements(ListNode head,int val，int depth){
    
    String depthString = generateDepthString(depth);
    System.out.print(depthString);
    System.out.print("Call: remove "+ val + " in " + head);
    
    if(head == null){
        System.out.print(depthString);
        return head;
    }
    head.next = removeElements(head.next,val,depth + 1);
    System.out.print(depthString);
     System.out.print("After: remove "+ val + " in " + res);
     
     ListNode ret;
     if(head.val == val){
         ret = res;
     }else{
         head.next = res;
         ret = head;
     }
     System.out.println(depthString);
    return ret;
    }
    
    private String generateDepthString(int depth){
        StringBuilder res = new StringBuilder();
        for(int i = 0; i< depth; i++){
            res.append("--");
        }
        return res.toString();
    }
    
  
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