While Large Language Models (LLMs) have significantly advanced code generation efficiency, they face inherent challenges in balancing performance and inference costs across diverse programming tasks. Dynamically selecting the optimal LLM based on task difficulty and resource constraints offers a promising approach to achieve an optimal balance between efficiency and performance. However, existing model selection methods are resource-intensive and often neglect cost efficiency. Moreover, these approaches rely on human-annotated difficulty labels that are frequently inaccessible in real-world settings and may not align with the LLM's own assessment of task difficulty. In this paper, we introduce AdaptiveLLM, a framework that dynamically selects optimal LLMs for a given coding task by automatically assessing task difficulty. Our framework first estimates task difficulty using Chain-of-Thought lengths generated by reasoning model, clusters these into three difficulty levels via k-means, and fine-tunes CodeBERT to embed difficulty-aware features. A trained XGBoost classifier then selects the best model for each problem, optimizing the performance-cost trade-off. Experimental results show that AdaptiveLLM achieves a 7.86% improvement in pass@1 score while reducing resource consumption by 88.9% compared to baseline method ComplexityNet. When compared to a single model, AdaptiveLLM demonstrates an approximately 15% accuracy improvement, while maintaining the same level of cost consumption. Apart from that, the difficulty assessment using CoT provides more reliable selection criteria than human evaluation. Our replication package is available at https://github.com/cjhCoder7/AdaptiveLLM.