The Plus One problem gives you a number represented as an array of digits and asks you to add 1 to it, returning the new digit array. This easy coding problem handles carry propagation from the least significant digit. The array and math interview pattern is the core.
Apple, Uber, Microsoft, Meta, Amazon, TikTok, Google, Bloomberg, and Adobe ask this as a quick implementation problem testing carry propagation and edge cases (all 9s needing an extra digit). Despite being easy, clean handling of the carry chain is the key skill.
Reverse carry propagation. Traverse from the last digit: if digit < 9, increment and return. If digit = 9, set to 0 and continue (carry propagates). If all digits were 9 (loop completes without early return), prepend 1: return [1] + [0]*n.
digits=[1,2,9]. Process from right: 9+1=10→set 9 to 0, carry. Next: 2+1=3≤9→set to 3, return. Result: [1,3,0].
digits=[9,9,9]: all become 0, prepend 1: [1,0,0,0].
Plus One tests carry propagation — a fundamental arithmetic operation. The early-return approach is cleaner than tracking a carry variable. Practice variants: "add two numbers represented as arrays," "increment number represented as linked list," "add one to number in place." All use the same right-to-left carry propagation pattern. The edge case (all 9s) is always worth mentioning in interviews.
