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public class Solution {
    public int maxProfit(int[] prices) {
        int hold1 = Integer.MIN_VALUE, hold2 = Integer.MIN_VALUE;
        int release1 = 0, release2 = 0;
        for(int i:prices){                              // Assume we only have 0 money at first
            release2 = Math.max(release2, hold2+i);     // The maximum if we've just sold 2nd stock so far.
            hold2    = Math.max(hold2,    release1-i);  // The maximum if we've just buy  2nd stock so far.
            release1 = Math.max(release1, hold1+i);     // The maximum if we've just sold 1nd stock so far.
            hold1    = Math.max(hold1,    -i);          // The maximum if we've just buy  1st stock so far. 
        }
        return release2; ///Since release1 is initiated as 0, so release2 will always higher than release1.
    }
}

public class Solution {
    public int maxProfit(int[] prices) {
        if (prices == null || prices.length < 2) return 0;
        int kk = 2;
        int len = prices.length;
        int[][] dp = new int[kk + 1][len];
        int res = 0;
        for (int k = 1; k <= kk; k++) {
            int tempMax = dp[k - 1][0] - prices[0];
            for (int i = 1; i < len; i++) {
                dp[k][i] = Math.max(dp[k][i - 1], prices[i] + tempMax);
                tempMax = Math.max(tempMax, dp[k - 1][i] - prices[i]);
                res = Math.max(res, dp[k][i]);
            }
        }
        return res;
    }
}