Please study the following problems related to backtracking:

1. http://acm.uva.es/p/v2/216.html

2. http://acm.uva.es/p/v7/732.html

3. http://acm.uva.es/p/v8/838.html

4. http://acm.uva.es/p/v100/10073.html

5. http://acm.uva.es/p/v103/10314.html

6. http://acm.uva.es/p/v1/197.html