What are common coding interview mistakes?

Most failed interview solutions are not conceptually wrong; they are incomplete. The candidate solves the main example, then misses the empty input, the duplicate value or the case where the answer is at the boundary. A short checklist before coding prevents a large share of these.

function firstIndex(values, target) {
  let left = 0;
  let right = values.length - 1;
  let result = -1;
  while (left <= right) {
    const mid = Math.floor((left + right) / 2);
    if (values[mid] === target) {
      result = mid;
      right = mid - 1;
    } else if (values[mid] < target) {
      left = mid + 1;
    } else {
      right = mid - 1;
    }
  }
  return result;
}

The checklist

• Clarify input size, value ranges, duplicates and whether the input may be mutated
• Test empty, single-element and all-equal inputs
• Check loop boundaries and inclusive versus exclusive ranges
• State time and space complexity after the code, not before
• Walk through the code with a small example instead of only describing it

Another common mistake is silently changing the problem. If the prompt asks for the first occurrence, returning any occurrence is wrong. If it asks for indices, sorting without preserving them is wrong. Repeat the requirement before you commit to an approach.