Constructive Algorithms
Build an object that meets given conditions by solving small cases, spotting the pattern, and using parity and invariants to know when it is impossible.
The problem
In a constructive problem you do not compute a number. You output an object: an array, a permutation, a string, a grid. Any object that satisfies the conditions is accepted, and a special program called a checker verifies it. If no such object exists, you print NO or -1.
Two skills are needed: finding a construction, and proving that none exists.
1 of 10