Experience record of Luogu's topic brushing

1. Transform ideas ： Variable angle ; The result is finally processed 100 10% 120
2. rounding ：

/* +0.5 Words , If greater than or equal to 0.5 Just move forward , Less than 0.5 Don't move forward , Then cast to int It's going to round down , Just round to an integer , This is one of the simplest rounding methods I think .  If you need to round to one place , then * How much and then divide   For example, one decimal place is *10/10.0  The last two *100/100.0  With one .0 It means floating point operation , So it won't be over int 了   Law two ： cout<<**floor**(s*10+0.5)/10.0;// return s The result rounded to the second decimal place  

4. double s=0.0000;// Please pay attention to the accuracy
sn Use double, Otherwise, the accuracy is not enough ……
Note the data types on both sides of the division （ Can't make it int, Otherwise, it is equal to div operation ）
5. Mathematical thought : Geometric series
6. Note that the distance entered may be decimal , So it has to be defined as a floating point number .
7. unsigned long long n,ans=0; // The pit is right here . The days are so big , Will the total distance be small ？？？
8.c Language has sprintf and sscanf, Corresponding stringstream Read and write of string stream

<stdlib.h> There is also the conversion between numeric values and strings iota and atoi function , Those who are interested can learn about it by themselves . use c Linguistic sprintf Than C++ High efficiency of string stream , But I still want to use C++ String , Why? ？ because C++ The extensibility and flexibility of strings are relatively high hhhh
9. Header file
count(ivec.begin() , ivec.end() , searchValue) This function is used to count the number of occurrences of a certain value within a certain range

10.2 The loveliness of a fish may be the same , So we need to discretize 
 Let's focus on discretization ：  For the following 4 Number , 10, 12, 9, 15  So the corresponding   The largest number is ： 2,3,1,4; That is to convert a large number into a small value （ One-to-one correspondence , People who have studied discrete mathematics should know this ）, As follows ： 10,12,9,15  First, the sequence subscript corresponding to the input ：1,2,3,4;  Put this 2 Put the rows in the structure ; Then the structure array follows the first line （10,12,9,15） Sort from small to large , The comparison function should be easy to write , The results are as follows ： 9,10,12,15; Corresponding to the next behavior 3,1,2,4;  And then put 9,10,12,15  Switch to  1,2,3,4, This structure is then constructed in accordance with the 2 OK, just line up again from small to large , obtain ： first line 2,3,1,4, The first 2 That's ok  1,2,3,4; This 2,3,1,4  Is the result of discretization , Note that there may be repeated numbers for this question , therefore   In the 9,10,12,15  Switch to  1,2,3,4 Pay attention to , If   yes  10,10, 12,15  Change it into 1,1,2,3;  The input data may be 0, So the input value should be added 1

11. The idea of the bucket ： cin>>a[i];// Read in g[a[i]]=1;// Assign a value of... In the set 1

12.//in[j]-‘a’: Read the position of the corresponding letter in , such as ’a’ Corresponding 0,‘b’ Corresponding 1,‘c’ Corresponding 2（0 Start ）;
//in[j]-‘a’+n: The corresponding position of the letter obtained by adding the transfer digit to the corresponding letter in the reading .
// for instance ’c’ This letter moves 3 position , That is the first. 2 Letters move to the right 3 individual , It's the fifth letter , namely ’f’
// Remainder 26 Is to prevent the number of bits from moving more than 26 Cause to blow up
// Plus ’a’, Restore the corresponding position to letters , Then type it out
11. Recurrence （ The initial conditions should be improved first ） Looking for a regular Think more about recursion
12. annular You can take the mold
13. The second one is —— It also improves the level of knowledge ——C++ Li real type if 0 Divide by a negative number to get -0 = = This operation is really explosive ！ At the last point, I made a mistake , A special judgment is required