Master Monotonic Sequences in Python: 7 Methods, Edge Cases & Interview Tips

If you've ever needed to verify if a list of stock prices, sensor readings, or ML training losses is trending consistently in one direction, checking for monotonicity is key. This is a common task in data pipelines, time-series analysis, and even tech interviews (shoutout to LeetCode 896!). What Is a Monotonic Sequence? A monotonic sequence is one that is entirely non-decreasing or entirely non-increasing . It never changes direction — no zigzagging. Main Types Non-decreasing Each element ≤ next element Allows equals Example: [1, 2, 2, 3, 5] Strictly increasing Each element < next element No equals allowed Example: [1, 3, 4, 8] Non-increasing Each element ≥ next element Allows equals Example: [9, 7, 7, 4, 2] Strictly decreasing Each element > next element No equals allowed Example: [10, 8, 5, 1] Most algorithm problems use non-strict (≤ or ≥) unless they explicitly say “strictly”. Quick Comparison Table Sequence Non-Decreasing Strictly Increasing Non-Increasing Strictly Decreasing [1,