[LeetCode] NO.21 Merge Two Sorted Lists

[题目] Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

[题目解析] 题目需要合并两个有序的链表，这是归并排序的中间步骤，数组和链表的合并如下。

    public int[] mergeArray(int[] arr1, int[] arr2){
    	int len1 = arr1.length;
    	int len2 = arr2.length;
    	int[] ret = new int[len1+len2];
    	int i=0,j=0,idx=0;
    	while(i < len1 && j < len2){
    		if(arr1[i] < arr2[j]){
    			ret[idx++] = arr1[i];
    			i++;
    		}else{
    			ret[idx++] = arr2[j];
    			j++;
    		}
    	}
    	while(i < len1){
    		ret[idx++] = arr1[i];
    		i++;
    	}
    	while(j < len2){
    		ret[idx++] = arr2[j];
    		j++;
    	}
    	return ret;
    }

　　

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(null == l1) return l2;
        if(null == l2) return l1;
        ListNode ret = new ListNode(0);
        ListNode head = ret;
        while(null != l1 && null != l2){
        	if(l1.val < l2.val){
        		ret.next = l1;
        		l1 = l1.next;
        	}else{
        		ret.next = l2;
        		l2 = l2.next;
        	}
        	ret = ret.next;
        }
        while(null != l1){
        	ret.next = l1;
        	l1 = l1.next;
        	ret = ret.next;
        }
        while(null != l2){
        	ret.next = l2;
        	l2 = l2.next;
        	ret = ret.next;
        }
        return head.next;
    }
}