Recursion Two Pointer — Solve Like Subproblems

Recursion two-pointer style: try a move at one end, recurse on the rest, backtrack if needed (classic for exhaustive search with pruning).

Try this in Python

def two_sum_sorted_all(nums: list[int], target: int, start: int) -> list[tuple[int, int]]:
    """Enumerate unique pairs >= start (educational)."""
    out: list[tuple[int, int]] = []
    i, j = start, len(nums) - 1
    while i < j:
        s = nums[i] + nums[j]
        if s == target:
            out.append((i, j))
            i += 1
            j -= 1
        elif s < target:
            i += 1
        else:
            j -= 1
    return out


print(two_sum_sorted_all([1, 1, 2, 3, 4], 5, 0))

Key takeaways

• Pure iterative here; recursion variant appears in DFS on grids.
• Mental model: recursion explores choices, pointers scan.
• Avoid exponential blow-up—add pruning.