151. Reverse Words in a String && 61. Rotate List && 189. Rotate Array

151. Reverse Words in a String

Given an input string, reverse the string word by word.

For example,
Given s = "the sky is blue",
return "blue is sky the".

• What constitutes a word?
  A sequence of non-space characters constitutes a word.
• Could the input string contain leading or trailing spaces?
  Yes. However, your reversed string should not contain leading or trailing spaces.
• How about multiple spaces between two words?
  Reduce them to a single space in the reversed string.

public class Solution {
    public String reverseWords(String s) {
        if(s.trim().equals(""))
            return "";

        String[] words = s.split(" ");
        StringBuilder sb = new StringBuilder();
        for(int i = words.length-1; i>=0; --i)
        {
            String trimed = words[i].trim();
            if(trimed.equals(""))
                continue;
            sb.append(trimed);
            sb.append(" ");
        }

        sb.setLength(sb.length()-1);
        return sb.toString();
    }
}

61. Rotate List

Given a list, rotate the list to the right by k places, where k is non-negative.

For example:
Given 1->2->3->4->5->NULL and k = 2,
return 4->5->1->2->3->NULL.

Linked List Two Pointers

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode rotateRight(ListNode head, int n) {
        if(head == null)
            return head;

        int len = 1;
        ListNode tail = head;
        while(tail.next!=null) {
            tail = tail.next;
            ++len;
        }

        int offset = n%len;
        if(offset == 0)
            return head;

        ListNode cut = head;
        for(int i = 0; i<len-offset-1;++i)
            cut = cut.next;

        ListNode newHead = cut.next;
        cut.next = null;
        tail.next = head;
        return newHead;
    }
}

189. Rotate Array

Rotate an array of n elements to the right by k steps.

For example, with n = 7 and k = 3, the array [1,2,3,4,5,6,7] is rotated to [5,6,7,1,2,3,4].

Note:
Try to come up as many solutions as you can, there are at least 3 different ways to solve this problem.

Hint:
Could you do it in-place with O(1) extra space?

public class Solution {
    public void rotate(int[] nums, int k) {
        int len = nums.length;
        k %= len;
        reverse(nums, 0, len-k-1);
        reverse(nums, len-k, len - 1);
        reverse(nums, 0, len - 1);
    }

    public void reverse(int[] nums, int start, int end) {
        while (start < end) {
            int temp = nums[start];
            nums[start] = nums[end];
            nums[end] = temp;
            ++start;
            --end;
        }
    }
}