MaxProfit

It is an easy exercise. The key point is making clear what you want to know during the iterating of the Array. In this question, the keys are current lowest point and current max profit

An array A consisting of N integers is given. It contains daily prices of a stock share for a period of N consecutive days. If a single share was bought on day P and sold on day Q, where 0 ≤ P ≤ Q < N, then the profit of such transaction is equal to A[Q] − A[P], provided that A[Q] ≥ A[P]. Otherwise, the transaction brings loss of A[P] − A[Q].

For example, consider the following array A consisting of six elements such that:

  A[0] = 23171
  A[1] = 21011
  A[2] = 21123
  A[3] = 21366
  A[4] = 21013
  A[5] = 21367

If a share was bought on day 0 and sold on day 2, a loss of 2048 would occur because A[2] − A[0] = 21123 − 23171 = −2048. If a share was bought on day 4 and sold on day 5, a profit of 354 would occur because A[5] − A[4] = 21367 − 21013 = 354. Maximum possible profit was 356. It would occur if a share was bought on day 1 and sold on day 5.

The code is here:

class Solution {
    public int solution(int[] A) {
        // write your code in C# 6.0 with .NET 4.5 (Mono)
        if(A.Length == 0) return 0;

        var curLowest = A[0];
        var curMaxProfit = 0;

        foreach(var a in A) {
            if(a < curLowest) {
                curLowest = a;
                continue;
            }

            curMaxProfit = Math.Max(curMaxProfit, a - curLowest);
        }

        return curMaxProfit;
    }
}