121. Best Time to Buy and Sell Stock ¶

Time: $O(n)$
Space: $O(1)$

C++JavaPythonJavaScript

class Solution {
 public:
  int maxProfit(vector<int>& prices) {
    int sellOne = 0;
    int holdOne = INT_MIN;

    for (const int price : prices) {
      sellOne = max(sellOne, holdOne + price);
      holdOne = max(holdOne, -price);
    }

    return sellOne;
  }
};

class Solution {
  public int maxProfit(int[] prices) {
    int sellOne = 0;
    int holdOne = Integer.MIN_VALUE;

    for (final int price : prices) {
      sellOne = Math.max(sellOne, holdOne + price);
      holdOne = Math.max(holdOne, -price);
    }

    return sellOne;
  }
}

class Solution:
  def maxProfit(self, prices: list[int]) -> int:
    sellOne = 0
    holdOne = -math.inf

    for price in prices:
      sellOne = max(sellOne, holdOne + price)
      holdOne = max(holdOne, -price)

    return sellOne

var maxProfit = function(prices) {
  let low = Infinity, best = 0;
  for (const p of prices) {
    low = Math.min(low, p);
    best = Math.max(best, p - low);
  }
  return best;
};