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31. Next Permutation

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  void nextPermutation(vector<int>& nums) {
    const int n = nums.size();

    // From back to front, find the first number < nums[i + 1].
    int i;
    for (i = n - 2; i >= 0; --i)
      if (nums[i] < nums[i + 1])
        break;

    // From back to front, find the first number > nums[i], swap it with
    // nums[i].
    if (i >= 0)
      for (int j = n - 1; j > i; --j)
        if (nums[j] > nums[i]) {
          swap(nums[i], nums[j]);
          break;
        }

    // Reverse nums[i + 1..n - 1].
    reverse(nums, i + 1, n - 1);
  }

 private:
  void reverse(vector<int>& nums, int l, int r) {
    while (l < r)
      swap(nums[l++], nums[r--]);
  }
};

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class Solution {
  public void nextPermutation(int[] nums) {
    final int n = nums.length;

    // From back to front, find the first number < nums[i + 1].
    int i;
    for (i = n - 2; i >= 0; --i)
      if (nums[i] < nums[i + 1])
        break;

    // From back to front, find the first number > nums[i], swap it with
    // nums[i].
    if (i >= 0)
      for (int j = n - 1; j > i; --j)
        if (nums[j] > nums[i]) {
          swap(nums, i, j);
          break;
        }

    // Reverse nums[i + 1..n - 1].
    reverse(nums, i + 1, n - 1);
  }

  private void reverse(int[] nums, int l, int r) {
    while (l < r)
      swap(nums, l++, 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 nextPermutation(self, nums: list[int]) -> None:
    n = len(nums)

    # From back to front, find the first number < nums[i + 1].
    i = n - 2
    while i >= 0:
      if nums[i] < nums[i + 1]:
        break
      i -= 1

    # From back to front, find the first number > nums[i], swap it with nums[i].
    if i >= 0:
      for j in range(n - 1, i, -1):
        if nums[j] > nums[i]:
          nums[i], nums[j] = nums[j], nums[i]
          break

    def reverse(nums: list[int], l: int, r: int) -> None:
      while l < r:
        nums[l], nums[r] = nums[r], nums[l]
        l += 1
        r -= 1

    # Reverse nums[i + 1..n - 1].
    reverse(nums, i + 1, len(nums) - 1)