The paper Constructions in combinatorics via neural networks by Adam Zsolt Wagner uses reinforcement learning algorithm and the deep cross-entropy method, to find explicit constructions and counterexamples to several open conjectures in extremal combinatorics and graph theory! Let me also mention the paper Refuting conjectures in extremal combinatorics via linear programming also by Adam Zsolt Wagner. (I thank Geordie Williamson for telling me about Adam's work.)