#1864 - Minimum Number of Swaps to Make the Binary String Alternating

Medium • View on LeetCode • View on GitHub

Problem Description

Given a binary string s, return the minimum number of character swaps to make it alternating, or -1 if it is impossible.

The string is called alternating if no two adjacent characters are equal. For example, the strings "010" and "1010" are alternating, while the string "0100" is not.

Any two characters may be swapped, even if they are not adjacent.

Solution

/**
 * @param {string} s
 * @return {number}
 */
var minSwaps = function(s) {
  const length = s.length;
  let ones = 0;
  let zeros = 0;

  for (const char of s) {
    if (char === '1') ones++;
    else zeros++;
  }

  if (Math.abs(ones - zeros) > 1) return -1;

  let mismatchesStartWithZero = 0;
  let mismatchesStartWithOne = 0;

  for (let i = 0; i < length; i++) {
    if (i % 2 === 0) {
      if (s[i] !== '0') mismatchesStartWithZero++;
      if (s[i] !== '1') mismatchesStartWithOne++;
    } else {
      if (s[i] !== '1') mismatchesStartWithZero++;
      if (s[i] !== '0') mismatchesStartWithOne++;
    }
  }

  if (ones === zeros) {
    return Math.min(mismatchesStartWithZero, mismatchesStartWithOne) >> 1;
  }

  return (ones > zeros ? mismatchesStartWithOne : mismatchesStartWithZero) >> 1;
};