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75. Sort Colors 👍

Approach 1: 3 pointers

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  void sortColors(vector<int>& nums) {
    int zero = -1;
    int one = -1;
    int two = -1;

    for (const int num : nums)
      if (num == 0) {
        nums[++two] = 2;
        nums[++one] = 1;
        nums[++zero] = 0;
      } else if (num == 1) {
        nums[++two] = 2;
        nums[++one] = 1;
      } else {
        nums[++two] = 2;
      }
  }
};

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class Solution {
  public void sortColors(int[] nums) {
    int zero = -1;
    int one = -1;
    int two = -1;

    for (final int num : nums)
      if (num == 0) {
        nums[++two] = 2;
        nums[++one] = 1;
        nums[++zero] = 0;
      } else if (num == 1) {
        nums[++two] = 2;
        nums[++one] = 1;
      } else {
        nums[++two] = 2;
      }
  }
}

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class Solution:
  def sortColors(self, nums: List[int]) -> None:
    zero = -1
    one = -1
    two = -1

    for num in nums:
      if num == 0:
        two += 1
        one += 1
        zero += 1
        nums[two] = 2
        nums[one] = 1
        nums[zero] = 0
      elif num == 1:
        two += 1
        one += 1
        nums[two] = 2
        nums[one] = 1
      else:
        two += 1
        nums[two] = 2

Approach 2: 2 pointers + swapping

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  void sortColors(vector<int>& nums) {
    int l = 0;                // Next 0 should be put in l
    int r = nums.size() - 1;  // Next 2 should be put in r

    for (int i = 0; i <= r;)
      if (nums[i] == 0)
        swap(nums[i++], nums[l++]);
      else if (nums[i] == 1)
        ++i;
      else
        // We may swap a 0 to index i, but we're still not sure whether
        // this 0 is put in the correct index, so we can't move pointer i
        swap(nums[i], nums[r--]);
  }
};

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class Solution {
  public void sortColors(int[] nums) {
    int l = 0;               // Next 0 should be put in l
    int r = nums.length - 1; // Next 2 should be put in r

    for (int i = 0; i <= r;)
      if (nums[i] == 0)
        swap(nums, i++, l++);
      else if (nums[i] == 1)
        ++i;
      else
        // We may swap a 0 to index i, but we're still not sure whether
        // this 0 is put in the correct index, so we can't move pointer i
        swap(nums, i, r--);
  }

  private void swap(int[] nums, int i, int j) {
    final int temp = nums[i];
    nums[i] = nums[j];
    nums[j] = temp;
  }
}

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class Solution:
  def sortColors(self, nums: List[int]) -> None:
    l = 0  # Next 0 should be put in l
    r = len(nums) - 1  # Next 2 should be put in r

    i = 0
    while i <= r:
      if nums[i] == 0:
        nums[i], nums[l] = nums[l], nums[i]
        i += 1
        l += 1
      elif nums[i] == 1:
        i += 1
      else:
        # We may swap a 0 to index i, but we're still not sure whether
        # this 0 is put in the correct index, so we can't move pointer i
        nums[i], nums[r] = nums[r], nums[i]
        r -= 1