Best Time to Buy and Sell Stock

One buy, one sell (buy before sell), maximize profit. [7,1,5,3,6,4] → buy at 1, sell at 6, profit 5. (LeetCode 121)

Approach: track the minimum price seen so far; for each day, the potential profit is price[i] - minPrice. Keep the best profit seen, and update the running minimum as you go.

int minimum = prices[0];
int maxProfit = 0;
int n = prices.length;

for(int i = 0; i < n; i++){
    int cost = prices[i] - minimum;
    maxProfit = Math.max(maxProfit, cost);
    minimum = Math.min(minimum, prices[i]);
}
return maxProfit;

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