Arrays

Array rotation

48. Rotate Image

Solution if from HERE

/*
 * clockwise rotate
 * first reverse up to down, then swap the symmetry 
 * 1 2 3     7 8 9     7 4 1
 * 4 5 6  => 4 5 6  => 8 5 2
 * 7 8 9     1 2 3     9 6 3
*/
class Solution 
{
public:
    void rotate(vector<vector<int>>& matrix) 
    {
        reverse(matrix.begin(), matrix.end());
        for(int i=0;i<matrix.size();++i)
            for(int j=i+1;j<matrix.size();++j)
                swap(matrix[i][j], matrix[j][i]);
    }
};

anticlockwise

/*
 * anticlockwise rotate
 * first reverse left to right, then swap the symmetry
 * 1 2 3     3 2 1     3 6 9
 * 4 5 6  => 6 5 4  => 2 5 8
 * 7 8 9     9 8 7     1 4 7
*/
void anti_rotate(vector<vector<int> > &matrix) {
    for (auto vi : matrix) reverse(vi.begin(), vi.end());
    for (int i = 0; i < matrix.size(); ++i) {
        for (int j = i + 1; j < matrix[i].size(); ++j)
            swap(matrix[i][j], matrix[j][i]);
    }
}

1477. Find Two Non-overlapping Sub-arrays Each With Target Sum

video from HERE

1060. Missing Element in Sorted Array

class Solution {
public:
    int helper(vector<int>& nums, int i, int k)
    {
        if (i == nums.size())
        {
            return nums.back() + k;
        }
        else if (nums[i] == nums[i-1])
        {
            return helper(nums, i+1, k);
        }
        else if (nums[i] - nums[i-1] - 1>=k)
        {
            return nums[i-1] + k;   
        }
        else
        {
            return helper(nums, i + 1, k - (nums[i] - nums[i-1] -1));
        }
    }
    int missingElement(vector<int>& nums, int k) 
    {
        return helper(nums, 1, k);
    }
};