Same Direction

Both pointers move left to right here instead of toward each other — one reads ahead (fast), one marks where the next "kept" value should go (slow).

int removeDuplicates(int *nums, int n) {
      if (n == 0) return 0;
      int slow = 0;
      for (int fast = 1; fast < n; fast++) {
          if (nums[fast] != nums[slow]) {
              slow++;
              nums[slow] = nums[fast];
          }
      }
      return slow + 1;   /* new length */
  }

slow only advances when something worth keeping shows up — it's a write cursor, not just a second reader. Move Zeroes and Is Subsequence are the same shape with a different keep-condition; Longest Mountain in Array needed while instead of if for the expansion steps, since a mountain's slopes can run for more than one element at a time.