# Eight queens problem (recursive backtracking)

```#include<stdio.h>
#include<stdlib.h>
int sum = 0;
int Danger(int r,int c, int(*a)[8])
{

for(int i=0;i<8;i++)
{
if(a[i][c])
return 0;
}
for(int i=r,j=c;i>=0&&j>=0;i--,j--)
{
if(a[i][j])
return 0;
}
for(int i=r,j=c;i>=0&&j<8;i--,j++)
{
if(a[i][j])
return 0;
}
return 1;

}
/*int Danger(int row, int j, int(*chess)[8]){
int flag1 = 1, flag2 = 1, flag3 = 1, flag4 = 1, flag5 = 1;
int ia, ib = 1, ic = 1, id = 1, ie = 1;
for(ia = 0; ia < 8; ia++){
if(*(*(chess + ia) + j) == 1){
flag1 = 0;
break;
}else flag1 = 1;
}

while((row - ib) != -1 && (j - ib) != -1){
if(*(*(chess + row - ib) + j - ib) == 1){
flag2 = 0;
break;
}else flag2 = 1;
ib++;
}

while((row + ic) != 8 && (j - ic) != -1){
if(*(*(chess + row + ic) + j - ic) == 1){
flag3 = 0;
break;
}else flag3 = 1;
ic++;
}

while((row - id) != -1 && (j + id) != 8){
if(*(*(chess + row - id) + j + id) == 1){
flag4 = 0;
}else flag4 = 1;
id++;
}

while((row + ie) != 8 && (j + ie) != 8){
if(*(*(chess + row + ie) + j + ie) == 1){
flag5 = 0;
break;
}else flag5 = 1;
ie++;
}
/*	printf("%d", flag1);
printf("%d", flag2);
printf("%d", flag3);
printf("%d", flag4);
printf("%d", flag5);
printf("\n");*/
//printf("/");
/*	if(flag1 && flag2 && flag3 && flag4 && flag5){
return 1;
}else return 0;
} */
void EQ(int row, int col, int(*chess)[8]){
/*	int chess[8][8];
for(int i = 0; i < 8; i++){
for(int j = 0; j < 8; j++){
chess[i][j] = pchess[i][j];
}
}*/
if(row == 8){
for(int i = 0; i < 8; i++){
for(int j = 0; j < 8; j++){
printf("%d ", *(*(chess + i) + j));
}
printf("\n");
}
printf("\n");
++sum;
return;
}
for(int j = 0; j < 8; j++ ){
if(Danger(row, j, chess) == 1){
*(*(chess + row) + j) = 1;
EQ(row + 1, col, chess);
*(*(chess + row) + j) = 0;
/*	for(int i = 0; i < 8; i++){
*(*(chess + row ) + i) = 0;
}
continue;*/
}

}

}

int main(){
int chess[8][8];
for(int i = 0; i < 8; i++){
for(int j = 0; j < 8; j++){
chess[i][j] = 0;
}
}
EQ(0, 8, chess);
printf("%d", sum);
return 0;
}
```

When the first code is implemented, the main logic is not considered clearly, and only output after one round of circulation. The problem is that there is no backtracking.

```for(int j = 0; j < 8; j++ ){
if(Danger(row, j, chess) == 1){
*(*(chess + row) + j) = 1;
EQ(row + 1, col, chess);
*(*(chess + row) + j) = 0;
```

## To flash back

Recurse to the last line. After the output is successful, the original position trace should be erased. Otherwise, it will affect the judgment of the Danger function

```*(*(chess + row) + j) = 0;
```
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Added by Visualant on Tue, 11 Feb 2020 17:55:36 +0200