Solve stock profit and vertical tree traversal

You have a 60-minute coding interview with two algorithm questions:

1. Max profit from one stock trade

• Input: an integer array prices, where prices[i] is the stock price on day i.
• Task: Choose one day to buy and a later day to sell to maximize profit. Return the maximum profit; if no profitable trade is possible, return 0.

2. Vertical traversal of a binary tree

• Input: the root of a binary tree.
• Define coordinates: root at (row=0, col=0), left child (row+1, col-1), right child (row+1, col+1).
• Task: Return the nodes grouped by vertical columns from leftmost col to rightmost.
  • Within each column, order nodes by row ascending (top to bottom).
  • If multiple nodes share the same (row, col), order those nodes by their value ascending.
• Output: a list of lists of node values following this ordering.