第七周总结

答案

package 算法;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class 简单表达式计算 {
	public static void main(String[] args) throws ScriptException, IOException {

		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

		String line;

		ScriptEngine scriptEngine = new ScriptEngineManager().getEngineByName("nashorn");

		while (!(line = in.readLine()).equals("END")) {

			System.out.println(String.valueOf(scriptEngine.eval(line)));

		}

		in.close();

	}

}

答题思路
这个题目我不太会做然后找找看到这个最少就感觉好强 然后这个感觉有点像偷懒用ScriptEnine的
getEngineByName(String shortName) 查找并创建一个给定名称的 ScriptEngine。方法调出js然后用js里的eval方法计算确实比较快捷方便

移除链表中的元素

public class 移除链表中的元素 {
	
	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;
        
    }
	
	public class ListNode {
		  int val;
		  ListNode next;
		  ListNode(int x) { val = x; }
	}
}

改进虚拟头结点
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
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;
    }
}

递归

public class Sum {
	public static int sum(int[] arr) {
		return sum(arr,0);
	}
	
	private static int sum(int arr[],int l) {
		if( l == arr.length) {
			return 0;
		}
		return arr[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));
	}
}

递归方法解决203号题

public class 移除链表中的元素3 {
	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;
		}
	}
	
	public class ListNode {
		  int val;
		  ListNode next;
		  ListNode(int x) { val = x; }
	}
}