Dungeon Game

The demons had captured the princess (P) and imprisoned her in the bottom-right corner of a dungeon. The dungeon consists of M x N rooms laid out in a 2D grid. Our valiant knight (K) was initially positioned in the top-left room and must fight his way through the dungeon to rescue the princess.

The knight has an initial health point represented by a positive integer. If at any point his health point drops to 0 or below, he dies immediately.

Some of the rooms are guarded by demons, so the knight loses health (negative integers) upon entering these rooms; other rooms are either empty (0's) or contain magic orbs that increase the knight's health (positive integers).

In order to reach the princess as quickly as possible, the knight decides to move only rightward or downward in each step.

Write a function to determine the knight's minimum initial health so that he is able to rescue the princess.

For example, given the dungeon below, the initial health of the knight must be at least 7 if he follows the optimal pathRIGHT-> RIGHT -> DOWN -> DOWN.

Notes:

• The knight's health has no upper bound.
• Any room can contain threats or power-ups, even the first room the knight enters and the bottom-right room where the princess is imprisoned.

Solution:

Start from bottom right, where the princess is imprisoned. Initialize the last row and column by recursively get the minimum life required before entering this cell. Comparing current minimum life to 1 using Math.min to ensure the knight's life never (shouldn't) drop below 1. Fill all cells by using DP, and finally return [0][0] to get the answer.

public int calculateMinimumHP(int[][] dungeon) {
    int row = dungeon.length - 1,
        col = dungeon[0].length - 1;
    int[][] board = new int[row + 1][col + 1];

    board[row][col] = Math.max(1 - dungeon[row][col], 1);
    //Initialize last col
    for (int i = row - 1; i >= 0; i--) {
        board[i][col] = Math.max(1, board[i + 1][col] - dungeon[i][col]);
    }
    //Initialize last row
    for (int i = col - 1; i >= 0; i--) {
        board[row][i] = Math.max(1, board[row][i + 1] - dungeon[row][i]);
    }

    for (int i = row - 1; i >= 0; i--) {
        for (int j = col - 1; j >= 0; j--) {
            int right   = Math.max(board[i][j + 1] - dungeon[i][j], 1),
                down    = Math.max(board[i + 1][j] - dungeon[i][j], 1);
            board[i][j] = Math.min(right, down);
        }
    }
    return board[0][0];
}