C language - Blue Bridge Cup - Snake filling

Title Description
As shown in the figure below , Xiao Ming uses from 1 A positive integer at the beginning “ Serpentine ” Fill an infinite matrix .
1 2 6 7 15 …
3 5 8 14 …
4 9 13 …
10 12 …
11 …
…
It is easy to see that the number in the second row and second column of the matrix is 5. Please calculate the number... In the matrix 20 Xing di 20 Column
What's the number of ？

1. Solution 1

The observation position is （1,1）,（2,2）,（3,3）（4,4）~~ The number of ,
It can be found that they are 1,5,13, 25~~, Find the law of these numbers and add 4* Subscript of the number , Such as ：5=1+4*1;13=5+4*2;25=13+4*3~~, From this, we can design a function to solve

2. Method 2

Rotate the matrix clockwise 45°, Then look for rules . You can find that the twentieth row and the twentieth column are the middle of the thirty ninth row after rotation , Then we can find the order of odd behavior by observing , Even number behavior in reverse order , Then there are several numbers in the first few lines , From this, we can find the last number in line 39 , subtracting （39-1） Half of , That is, the number in the middle of the thirty-nine lines

3. The answer for 761 

4. The attached code is as follows  

1.

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
    int i = 0;
    int a = 1;
    for (i = 1; i <= 19; i++)
    {
        a = a + i * 4;
    }
    printf("%d\n", a);
    return 0;
}

2.

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
    int i = 0;
    int a = 0;
    int b = 0;
    a = 20 * 2 - 1;
    for (i = 1; i <= a; i++)
    {
        b += i;
    }
    b = b - (a - 1) / 2;
    printf("%d\n", b);
    return 0;
}