The Flood Stone of Mill Ford

The miller at Mill Ford keeps a flood stone by the wheel: a line carved in the rock, and beside it a tally of the river's height. Each morning a notch count is written down — a positive number for notches above the line, a negative number for notches below it, and zero when the water sits exactly on the line.

The miller wants a second column beside the first: for each morning, the highest reading recorded so far, counting that morning and every morning before it.

Write highWaterMarks(marks).

• marks is a list of integers, in the order the readings were taken. It may be empty.
• Return a new list of the same length, where the value at each position is the largest value found at that position or any earlier position in marks.
• For an empty list of readings, return an empty list.