303. Range Sum Query - Immutable

303 題目截圖

class NumArray {
    private int[] sumSoFar;
    
    public NumArray(int[] nums) {
        sumSoFar = new int[nums.length + 1];
        for (int i = 0; i < nums.length; i++) {
            sumSoFar[i + 1] = sumSoFar[i] + nums[i];
        }
    }
    
    public int sumRange(int i, int j) {
        return sumSoFar[j + 1] - sumSoFar[i];
    }
}

304 題目截圖

class NumMatrix {
    private int[][] dpSum;
    
    public NumMatrix(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0] == null || matrix[0].length == 0) {
            return;
        }
        
        int rows = matrix.length;
        int cols = matrix[0].length;
        dpSum = new int[rows + 1][cols + 1];
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                dpSum[i+1][j+1] = dpSum[i+1][j] + dpSum[i][j+1] - dpSum[i][j] + matrix[i][j];
            }
        }
    }
    
    public int sumRegion(int row1, int col1, int row2, int col2) {
        return dpSum[row2+1][col2+1] - dpSum[row1][col2+1] - dpSum[row2+1][col1] + dpSum[row1][col1];
    }
}