feat: add merge intervals algorithm

greedy_methods/merge_intervals.py

@@ -0,0 +1,93 @@
+"""
+Given a collection of intervals, merge all overlapping intervals.
+
+This is a classic greedy algorithm problem. The key insight is that after sorting
+the intervals by their start time, we can merge overlapping intervals in a single
+pass by comparing each interval's start with the previous interval's end.
+
+Reference: https://en.wikipedia.org/wiki/Interval_scheduling#Interval_Partitioning
+
+For doctests run the following command:
+python -m doctest -v merge_intervals.py
+"""
+
+
+def merge_intervals(intervals: list[list[int]]) -> list[list[int]]:
+    """
+    Merge all overlapping intervals and return the non-overlapping intervals
+    that cover all the intervals in the input.
+
+    Args:
+        intervals: A list of intervals where each interval is [start, end].
+
+    Returns:
+        A list of merged non-overlapping intervals sorted by start time.
+
+    Examples:
+    >>> merge_intervals([[1, 3], [2, 6], [8, 10], [15, 18]])
+    [[1, 6], [8, 10], [15, 18]]
+    >>> merge_intervals([[1, 4], [4, 5]])
+    [[1, 5]]
+    >>> merge_intervals([[1, 4], [0, 4]])
+    [[0, 4]]
+    >>> merge_intervals([[1, 10], [2, 3], [4, 5], [6, 7]])
+    [[1, 10]]
+    >>> merge_intervals([[1, 2]])
+    [[1, 2]]
+    >>> merge_intervals([])
+    []
+    >>> merge_intervals([[3, 5], [1, 2], [6, 8], [2, 4]])
+    [[1, 5], [6, 8]]
+    >>> merge_intervals([[1, 3], [2, 6], [8, 10], [15, 18], [17, 20]])
+    [[1, 6], [8, 10], [15, 20]]
+    >>> merge_intervals([[1, 1]])
+    [[1, 1]]
+    >>> merge_intervals("not a list")
+    Traceback (most recent call last):
+        ...
+    TypeError: intervals must be a list of intervals
+    >>> merge_intervals([[1, 2], "not an interval"])
+    Traceback (most recent call last):
+        ...
+    TypeError: each interval must be a list of two integers
+    >>> merge_intervals([[1, 2], [3]])
+    Traceback (most recent call last):
+        ...
+    TypeError: each interval must be a list of two integers
+    >>> merge_intervals([[2, 1]])
+    Traceback (most recent call last):
+        ...
+    ValueError: interval start must not exceed end: [2, 1]
+    """
+    if not isinstance(intervals, list):
+        raise TypeError("intervals must be a list of intervals")
+
+    for interval in intervals:
+        if not isinstance(interval, list) or len(interval) != 2:
+            raise TypeError("each interval must be a list of two integers")
+        if interval[0] > interval[1]:
+            msg = f"interval start must not exceed end: {interval}"
+            raise ValueError(msg)
+
+    intervals.sort(key=lambda x: x[0])
+
+    merged: list[list[int]] = []
+    for interval in intervals:
+        # If merged is empty or current interval does not overlap with previous
+        if not merged or merged[-1][1] < interval[0]:
+            merged.append(interval)
+        else:
+            # There is overlap, so merge the current and previous intervals
+            merged[-1][1] = max(merged[-1][1], interval[1])
+
+    return merged
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
+    example = [[1, 3], [2, 6], [8, 10], [15, 18]]
+    print(f"Intervals: {example}")
+    print(f"Merged: {merge_intervals(example)}")