public int uniquePaths(int m, int n) {
		if (m == 0 || n == 0) {
			return 0;
		}
		// 1. initialize
		int[][] sum = new int[m][n];
		for (int i = 0; i < m; i++) {
			sum[i][0] = 1;
		}
		for (int i = 0; i < n; i++) {
			sum[0][i] = 1;
		}
		for (int i = 1; i < m; i++) {
			for (int j = 1; j < n; j++) {
				sum[i][j] = sum[i - 1][j] + sum[i][j - 1];
				// go down (j - 1) or go right (i - 1)
			}
		}
		return sum[m - 1][n - 1];
	}