Given an array of stock prices, find the maximum profit by buying and selling multiple times.
Example 1:
Input: prices = [7, 1, 5, 3, 6, 4]
Output: 7
Explanation: Buy on day 2 (price=1), sell on day 3 (price=5), profit=4.
Buy on day 4 (price=3), sell on day 5 (price=6), profit=3.
Total profit = 4 + 3 = 7.
Example 2:
Input: prices = [1, 2, 3, 4, 5]
Output: 4
Explanation: Buy on day 1 (price=1), sell on day 5 (price=5), profit=4.