LeetCode 238 Product of Array Except Self - Space Optimization

The $O(1)$ extra space solution uses the output array for prefix products, then multiplies in suffix products in a second pass. First pass: $answer[i] =$ product of all elements before $i$.

Start with $answer[0] = 1$, then $answer[i] = answer[i-1] \times nums[i-1]$. Second pass: track $suffix$ (product of elements after current index). For each $i$ from $n-1$ down to $0$: $answer[i] = answer[i] \times suffix$, then $suffix = suffix \times nums[i]$. After both passes, $answer[i]$ = product of all elements except $nums[i]$. Time: $O(n)$, Space: $O(1)$ excluding output.