Question

两个人（A，B）参与一个游戏，规则如下：

1）一个随机的整数数列有偶数个数,a1,a2,…a2n

2）A 先从数列取数，但只能从两头取,a1 or a2n

3)然后Ｂ取数，也是只能从剩下的两头取，依此类推，两个人轮流，都只能从两头取

Output the max sum of numbers that player 1 is going to get.

Solution

This question can be solved with DP by using the following 3 equations:

sum[i][j] sum of number from i to j
val[i][j]: the max value that can be obtained if only the range [i,j] is left and you take the move first
pos[][] is related to val[][], as it denotes whether we take i or j

After we got sum[][] array fully filled up, the transition equation of val[][] looks like:

val[x][y] = sum[x][y] - MIN( val[x+1][y], val[x][y-1] )

As for pos[][]:

pos[x][y] = x  if val[x+1][y] < val[x][y-1]
            y  otherwise

Code

public int getMaxSumPlayerOne(int[] input) {
	int len = input.length;
	// sum[i][j] sum of number from i to j
	int[][] sum = new int[len][len];
	// val[i][j]: the max value that can be obtained if only
	// the range [i,j] is left and you take the move first
	int[][] val = new int[len][len];
	// pos is related to val, as it denotes whether we take i or j
	int[][] pos = new int[len][len];

	// 1. fill in sum[][]
	for (int x = 0; x < len; x++) {
		for (int y = x; y < len; y++) {
			if (x == y) {
				sum[x][y] = input[x];
			} else if (x == 0) {
				// fill up the first row in the sum[][] dp array
				sum[x][y] = sum[x][y - 1] + input[y];
			} else {
				// starting from the 2nd row, it's gonna make use of 1st row
				// x >= 1
				sum[x][y] = sum[0][y] - sum[0][x - 1];
			}
		}
	}

	// 2. fill in val[][] and pos[][]
	for (int x = len - 1; x >= 0; x--) {
		for (int y = x; y < len; y++) {
			if (x == y) {
				val[x][y] = input[x];
				pos[x][y] = x;
			} else {
				// when I choose either x or y, I look at what the opponent
				// is gonna get after my move, then chose the smaller
				// v[][] for the remaining part
				val[x][y] = sum[x][y] - Math.min(val[x + 1][y], val[x][y - 1]);
				pos[x][y] = val[x + 1][y] < val[x][y - 1] ? x : y;
			}
		}
	}
	this.printSteps(pos, len);
	return val[0][len - 1];
}

Testing

Input {1, 4, 3, 2}
Player 1 take position 3
Player 2 take position 2
Player 1 take position 1
Player 2 take position 0
max sum for player 1 is 6

Input {7, 5, 2, 6, 9, 8, 3, 4}
Player 1 take position 0
Player 2 take position 1
Player 1 take position 7
Player 2 take position 6
Player 1 take position 5
Player 2 take position 4
Player 1 take position 3
Player 2 take position 2
max sum for player 1 is 25