Ada-R1: Hybrid-CoT via Bi-Level Adaptive Reasoning Optimization - Luo, Haotian; He, Haiying; Wang, Yibo; Yang, Jinluan; Liu, Rui; Tan, Naiqiang; Cao, Xiaochun; Tao, Dacheng; Shen, Li (Advances in Neural Information Processing Systems, 2025)