Perform String Shifts

Perform String Shifts

What is this problem about?

The Perform String Shifts problem gives you a string and a list of shift operations (direction: 0=left, 1=right, amount). Apply all shifts to the string and return the result. This easy coding problem has an O(n) insight: accumulate the total net shift and perform a single rotation. The array, math, and string interview pattern is demonstrated.

Why is this asked in interviews?

Goldman Sachs asks this to test the key optimization insight: instead of performing each rotation individually (O(nk)), sum all left and right shifts to get the net shift, then do a single string rotation. This reduces O(nk) to O(n).

Algorithmic pattern used

Net shift accumulation + single rotation. For each shift [dir, amt]: if dir=0 (left), subtract amt; if dir=1 (right), add amt. Compute net_shift % n (handle negative modulo). Rotate: right shift by net_shift ≡ s[-net_shift:] + s[:-net_shift].

Example explanation

s="abcdef", shifts=[[1,1],[1,1],[0,2],[1,3]]. Net = +1+1-2+3 = 3. net_shift = 3. Right rotate by 3: "def" + "abc" = "defabc".

Common mistakes candidates make

• Performing each rotation separately (O(n * total_shifts)).
• Incorrect modulo for negative shift (must add n to get positive: net % n handles this in Python, but not in C++/Java for negative).
• Off-by-one in string rotation: right shift by k = s[n-k:] + s[:n-k].
• Not handling net_shift=0 case (string unchanged).

Interview preparation tip

String rotation problems always reduce to a single rotation by accumulating the net shift. The net shift approach is the critical optimization. Practice string rotation: s[k:] + s[:k] for left rotation, s[-k:] + s[:-k] for right rotation. Be careful with negative modulo in different languages — always ensure the final shift is in [0, n-1].