Pointer Manipulation Target: 15s
Converging: start from both ends, narrow down. Same-direction: fast pointer reads, slow pointer writes. Both are O(n).
TWO POINTERS — CONVERGING vs SAME-DIRECTION
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CONVERGING (opposite ends → middle):
l, r = 0, n - 1
while l < r:
if condition: l += 1
else: r -= 1
→ Sorted array: two sum, 3sum, container with most water
→ Palindrome check
→ Partitioning (Dutch national flag)
SAME-DIRECTION (both move forward):
slow, fast = 0, 0
while fast < n:
if condition:
a[slow] = a[fast]
slow += 1
fast += 1
→ Remove duplicates in-place
→ Move zeros to end
→ Linked list: fast/slow for cycle detection, middle
DECISION:
Sorted + pair sum? → converging
Palindrome? → converging
Remove/compact? → same-direction (read/write)
Cycle in linked list? → same-direction (fast/slow)
Merge two sorted? → same-direction (one per array)Type it from memory. Go.