121-best-time-to-buy-and-sell-stock.java

class Solution {
    public int maxProfit(int[] prices) {
        int res = 0;
        int left = 0;
        for (int right = 0; right < prices.length; right++) {
            if (prices[right] > prices[left]) {
                res = Math.max(res, prices[right] - prices[left]);
            } else {
                left = right;
            }
        }
        return res;
    }
}

// time O(n)
// space O(1)
// using array and two pointers same direction and left ptr to record and greedy