Rehabilitation type force deduction brush question notes D1

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Code section

topic 1

topic 2

  topic 3

java Small knowledge part

topic 1

topic 2

topic 3

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Code section

topic 1

1. Sum of two numbers

The difficulty is simple 13081 Switch to English to receive dynamic feedback

Given an array of integers  nums  And an integer target value  target, Please find... In the array   And is the target value  target  the   Two   Integers , And return their array subscripts .

You can assume that each input corresponds to only one answer . however , The same element in the array cannot be repeated in the answer .

You can return the answers in any order .

         Self solution ：

class Solution {
    public int[] twoSum(int[] nums, int target) {
        for(int i = 0 ;i < nums.length ; i++)
        {
            int flag;
            flag = target - nums[i];
            for(int j = i+1 ;j < nums.length ; j++)
            {
                if(nums[j] == flag) return new int[] {i,j};
                else continue;
            }
        }
        return null;
    }
}

         Double cycle , Time complexity . Bubble like thinking .

topic 2

2. Addition of two numbers

Medium difficulty 7312 Switch to English to receive dynamic feedback

Here are two for you   Non empty   The linked list of , Represents two nonnegative integers . Each of them is based on   The reverse   Stored in , And each node can only store   a   Numbers .

Please add up the two numbers , And returns a linked list representing sum in the same form .

You can assume that in addition to the numbers 0 outside , Neither of these numbers 0  start .

         Self solution ：

/**
 * 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 addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode root = new ListNode();
        ListNode result = root;
        int andre , catchre = 0 , nextre = 0;
        while(l1 != null || l2 != null || catchre == 1)
        {
            int l1val = (l1 != null)? l1.val:0;
            int l2val = (l2 != null)? l2.val:0;
            if(l1 ==null && l2 != null)
            {
                andre = l2val + catchre;
                nextre = andre + (-10)*((andre >= 10)?1:0);
                catchre = ((andre >= 10)?1:0);
            }
            else if(l2 ==null && l1 != null)
            {
                andre = l1val + catchre;
                nextre = andre + (-10)*((andre >= 10)?1:0);
                catchre = ((andre >= 10)?1:0);
            }
            else if(l1 == null &&l2 == null && catchre == 1)
            {
                andre = catchre;
                nextre = andre + (-10)*((andre >= 10)?1:0);
                catchre = ((andre >= 10)?1:0);
            }
            else
            {
                    andre = l1val + l2val + catchre;
                    nextre = andre + (-10)*((andre >= 10)?1:0);
                    catchre = ((andre >= 10)?1:0);
            }
            ListNode Next = new ListNode (nextre);
            result.next = Next;
            result = Next;
            if(l1 != null) l1 = l1.next;
            if(l2 != null) l2 = l2.next;
        }
        return root.next;

    }
}

        Use unit adder thinking . Set up andre Direct addition result ,catchre Carry mark （ The carry flag bit is cf, Should be carry flag, I remember wrong ）,nextre The value left after processing .

        The core code snippet is as follows , That is, the processing of addition results and the generation of carry flag bits .

andre = l1val + l2val + catchre;
nextre = andre + (-10)*((andre >= 10)?1:0);
catchre = ((andre >= 10)?1:0);

  topic 3

3. Longest substring without repeating characters

Medium difficulty 6697 Switch to English to receive dynamic feedback

Given a string  s , Please find out that there are no duplicate characters in it   Longest substrings   The length of .

        Learn from the code of the great God , A lot of comments are added ：

class Solution {
    public int lengthOfLongestSubstring(String s) {

        int [] last =new int[128]; // Where the character last appeared 
        for(int i = 0 ; i < 128 ; i++)
        {
            last[i] = -1; // initialization 
        }
        int res = 0; // Output value 
        int start = 0; // Starting position 
        char [] S = s.toCharArray(); //string Turn array 
        for(int i = 0; i < s.length() ; i++) // Is a loop over the original string 
        {
            int index = S[i]; // Take out the character to be positioned 
            start = Math.max (start , last[index] + 1); 
            //start The value is current. start Value and the last occurrence position of the character plus one to get the maximum value 
            res = Math.max (res , i - start + 1);
            //res The value is current. res The value of the space between the value and the character is the maximum 
            last[index] = i; // Character last The value is updated to the current character position 
        }
        return res;
    }
}

         According to the original code comments , Using the idea of window . It can be understood that there is a box always in the search position , When a duplicate character appears, the box is updated to the next digit after the current character .

java Small knowledge part

topic 1

        Take the array length method ： Array .length.

topic 2

        To operate on a linked list, you need to first operate on an original linked list （root） Define and allocate memory , Then define a linked list to be operated （result）, Finally, return to the original linked list （root）.

ListNode root = new ListNode();
ListNode result = root;

return root.next;

         The linked list operation needs to generate new nodes , Then link to the existing list . Then let the pointer of the linked list climb backward .

ListNode Next = new ListNode (nextre);
result.next = Next;
result = Next;

        boolean Type numbers cannot participate in operations , Need to use “(andre >= 10)?1:0” The method is transformed into 1 or 0, Here 1、0 It can also be replaced with other numbers .

         Carry out linked list node = List nodes .next when , First judge whether it is empty , namely “if( node != null) node  = node .next;”.

topic 3

        String turn char Array ：

char [] S = s.toCharArray();

        Take the maximum function ：Math.max.