leetcode Top100 之MaximumSubarray/MergeTwoSortedLists

这是leetcode top 100中的21、53题

第21题，将两个有序的链表，拼接在一起。

package leetcodeJava.array;


public class top100_21_MergeTwoSortedLists {
	/**
	 * Definition for singly-linked list.
	 * public class ListNode {
	 *     int val;
	 *     ListNode next;
	 *     ListNode(int x) { val = x; }
	 * }
	 */
	public class ListNode{
		int val;
		ListNode next;
		public ListNode(int x){
			val=x;
		}
	}
	
	class Soulution1{
		//递归的形式，非常简洁
		public ListNode mergeTwoLists(ListNode l1, ListNode l2){
			if (l1==null&&l2==null) return null;
			if (l1==null) return l2;
			if (l2==null) return l1;
			if (l1.val<l2.val){
				l1.next=mergeTwoLists(l1.next, l2);
						return l1;
			}
			
			else {
				l2.next=mergeTwoLists(l1, l2.next);
				return l2;
			}
		}
}
	
	
	
	class Solution {
		//非递归形式，其中注意以下两点
	    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
	    	ListNode newNode=new ListNode(0);
	    	ListNode head=newNode;	
	    	//1,需要用一个指针一直指向在移动的newNode,这样最后返回head.next就是整个赋值后的链表
	    	//而不是返回newNode,因为返回的就是一部分，而不是整个链表了
	    	
	    	while(l1!=null && l2!=null){
	    		if (l1.val<l2.val){
		    		newNode.next=l1;
		    		l1=l1.next;	
		    	}
	    		else {
					newNode.next=l2;
					l2=l2.next;
				}
	    		newNode=newNode.next;
	    	}
	    	
	    	if (l1==null){			
	    		//也可以用while循环，但是在直接把l2的剩余部分连接上去后，必须return结束了，就像solution3中那样
	    		//不然会陷入死循环				
	    		newNode.next=l2;
	    		
	    	}
	    	else {
				newNode.next=l1;
			}
	    	
			return head.next;
	        
	    }
	}
	
	class Solution3{
	    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
	    	ListNode newNode=new ListNode(0);
	    	ListNode head=newNode;
	    	
	    	while(l1!=null && l2!=null){
	    		if (l1.val<l2.val){
		    		newNode.next=l1;
		    		l1=l1.next;	
		    	}
	    		else {
					newNode.next=l2;
					l2=l2.next;
				}
	    		newNode=newNode.next;
	    	}
	    	
	    	while (l1==null){
	    		newNode.next=l2;
                return head.next;
	    		
	    	}
	    	
	    	while (l2==null) {
				newNode.next=l1;
                return head.next;
			}
	    	
			return head.next;
	        
	    }
	}

}

这是第53题，给定一个数组，求这个数组的子数组的最大和。

Find the contiguous subarray within an array (containing at least one number) which has the largest sum.

For example, given the array [-2,1,-3,4,-1,2,1,-5,4],
the contiguous subarray [4,-1,2,1] has the largest sum = 6.

package leetcodeJava.array;

public class top100_53_MaximumSubarray {
	
	/*
	 * Find the contiguous subarray within an array (containing at least one number) which has the largest sum.

		For example, given the array [-2,1,-3,4,-1,2,1,-5,4],
		the contiguous subarray [4,-1,2,1] has the largest sum = 6.
		*/
	
	public static int maxSubarray(int [] nums){
		
		int A=nums.length;
		int[] dp=new int[A];
		dp[0]=nums[0];
		int maxSum=dp[0];
		for (int i=1;i<A;i++){
			dp[i]=nums[i]+(dp[i-1]>0 ? dp[i-1]:0);
			maxSum=Math.max(maxSum, dp[i]);
		}
		
		return maxSum;
	}
	
	public static void main(String[] args){
		int [] arr={-2,1,-3,4,-1,2,1,-5,4};
		System.out.println(maxSubarray(arr));
		
	}

}