We define three states: hold (holding a stock), sold (just sold), rest (cooldown). We transition between states each day.

1. hold[i] = max(hold[i-1], rest[i-1] - prices[i]) — keep holding or buy.
2. sold[i] = hold[i-1] + prices[i] — sell the stock.
3. rest[i] = max(rest[i-1], sold[i-1]) — do nothing or come from cooldown.
4. Return max(sold[n-1], rest[n-1]).

• Time Complexity: O(N).
• Space Complexity: O(1).

def max_profit(prices):
    if not prices:
        return 0
    
    hold = -prices[0]
    sold = 0
    rest = 0
    
    for i in range(1, len(prices)):
        prev_hold = hold
        hold = max(hold, rest - prices[i])
        rest = max(rest, sold)
        sold = prev_hold + prices[i]
    
    return max(sold, rest)