剑指offer 21、22、24、25

原题链接

class Solution {
    public int[] exchange(int[] nums) {
        int i = 0, j = nums.length - 1, tmp = 0;
        while(i < j){
            while(i < j && (nums[i] & 1) == 1) i++;
            while(i < j && (nums[j] & 1) == 0) j--;
            tmp = nums[i];
            nums[i] = nums[j];
            nums[j] = tmp;
        }
        return nums;
    }
}
//进行与运算：相同为0，不同为1
class Solution {
    public int[] exchange(int[] nums) {
        int i = 0, j = nums.length - 1, tmp;
        while(i < j) {
            //若与1进行与运算后值为1，则说明是奇数
            while(i < j && (nums[i] & 1) == 1) i++;
            //若与1进行与运算后值为0，则说明是偶数
            while(i < j && (nums[j] & 1) == 0) j--;
            tmp = nums[i];
            nums[i] = nums[j];
            nums[j] = tmp;
        }
        return nums;
    }
}

原题链接

class Solution {
    public ListNode getKthFromEnd(ListNode head, int k) {
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode fast = dummy;
        ListNode slow = dummy;
        while(k-- > 0){
            fast = fast.next;
        }
        while(fast != null){
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }
}

原题链接

1. 要自定义三个节点，pre、cur、tmp

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null;
        ListNode cur = head;
        ListNode tmp = null;
        while(cur != null){
            tmp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = tmp;
        }
        return pre;
    }
}

原题链接

1. 使用递归的思想，不断比较
2. 若其中一个节点为null，则直接返回另外一个节点
3. 若L1.val < L2.val 则需要比较L1中的下一个节点，即 L1.next 和 L2的关系，层层调用然后返回

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        else if(l2 == null) return l1;
        else if(l1.val < l2.val){
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        }else{
            l2.next = mergeTwoLists(l2.next, l1);
            return l2;
        }
    }
}