121.买卖股票的最佳时机

贪心

时间复杂度$O(n)$

空间复杂度$O(1)$

class Solution {
    public int maxProfit(int[] prices) {
        int ans = 0, preMin = prices[0];
        for (int i = 1; i < prices.length; ++i) {
            ans = Math.max(ans, prices[i] - preMin);
            preMin = Math.min(preMin, prices[i]);
        }
        return ans;
    }
}