Confused...

Back-end part:

int[][] matrix = new int[n][m];
int[] array = new int[n*m];

traversMatrix(matrix);
traversArray(array);

Client part:

function traversMatrix(matrix : int[][])
  for (i..n)
    for (j..m)
      doActions();

function traversArray(array : int[])
  for (i..array.size())
      doActions();

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Time Complexity for traversMatrix: Quadratic 
Time Complexity for traversArray : Linear?

Why?