73. Set Matrix Zeroes

Problem statement:

Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in place.

Follow up:

Did you use extra space?
A straight forward solution using O(mn) space is probably a bad idea.
A simple improvement uses O(m + n) space, but still not the best solution.
Could you devise a constant space solution?

Solutions:

In follow up, O(mn) and O(m+n) is not the best solution, I use O(n+1) space complexitiy to solve the problem.

First, 

I set up an bool flat to indicate if current row is selected to reset to 0.

I set up an bool array to indicate if current col is selected to reset to 0.

Second, Set up a two-level loop to enumerate all element to pick the row index and col index

Third, reset all the element to 0.

    void setZeroes(vector<vector<int>>& matrix) {
        if(matrix.empty()){
            return;
        }
        
        bool row_reset = false;
        vector<bool> col_reset(matrix[0].size(), false);
        
        // row index to reset to 0
        vector<int> row_idx;
        // col index to reset to 0
        vector<int> col_idx;   
        
        for(vector<int>::size_type ix = 0; ix < matrix.size(); ix++){
            row_reset = false;
            for(vector<int>::size_type iy = 0; iy < matrix[0].size(); iy++){
                if(matrix[ix][iy] == 0){
                    // current row is already selected to reset to 0
                    if(!row_reset){
                        row_idx.push_back(ix);
                        // avoid duplicate push back
                        row_reset = true;
                    }
                    // current col is already selected to reset to 0
                    if(!col_reset[iy]){
                        col_idx.push_back(iy);
                        // avoid duplicate push back
                        col_reset[iy] = true;
                    }
                }
            }
        }
        
        // reset selected row to 0
        for(vector<int>::size_type ix = 0; ix < row_idx.size(); ix++){
            for(vector<int>::size_type iy = 0; iy < matrix[row_idx[ix]].size(); iy++){
                matrix[row_idx[ix]][iy] = 0;
            }
        }
        // reset selected col to 0
        for(vector<int>::size_type ix = 0; ix < col_idx.size(); ix++){
            for(vector<int>::size_type iy = 0; iy < matrix.size(); iy++){
                matrix[iy][col_idx[ix]] = 0;
            }
        }
        return;
    }