LeetCode热题100道——Day01

1.两数之和

class Solution {
    public int[] twoSum(int[] nums, int target) {

        Map<Integer, Integer> map = new HashMap<>();

        for(int i = 0; i < nums.length; i++){
            int searchNum = target - nums[i];

            if(map.containsKey(searchNum)){
                return new int[]{map.get(searchNum), i};
            }
            map.put(nums[i], i);
        }

        return null;
    }
}

15.三数之和

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        
        //数组长度
        int len = nums.length;
        
        //结果集
        List<List<Integer>> result = new ArrayList<>();
        
        //对数组进行排序
        Arrays.sort(nums);
        
        //遍历每一个元素
        for(int i = 0; i < len; i++){
            
            //当前数大于0直接退出循环
            if(nums[i] > 0) break;
            
            //如果当前数重复直接跳过
            if(i > 0 && nums[i] == nums[i-1]) continue;
            
            //左指针
            int lp = i + 1;
            //右指针
            int rp = len - 1;
            
            //当左右指针不重叠时，可以移动指针
            while(lp < rp){
                
                int sum = nums[i] + nums[lp] + nums[rp];
                
                if(sum == 0){
                    result.add(Arrays.asList(nums[i], nums[lp], nums[rp]));
                    lp++;
                    rp--;
                    //跳过连续重复的元素
                    while(lp < rp && nums[lp] == nums[lp-1]) lp++;
                    while(lp < rp && nums[rp] == nums[rp+1]) rp--;
                }else if(sum < 0){
                    lp++;
                }else{
                    rp--;
                }
            }
        }
        
        return result;
        
    }
}

19.删除链表的倒数第N个结点

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {

        ListNode sentinel = new ListNode(-1, head);
        ListNode front = sentinel;
        ListNode back = sentinel;

        for(int i = 0; i < n+1; i++){
            front = front.next;
        }

        while(front != null){
            front = front.next;
            back = back.next;
        }

        back.next = back.next.next;

        return sentinel.next;
    }
}