range-sum-query-2d-immutable.js

/**
 * @param {number[][]} matrix
 */
var NumMatrix = function (matrix) {
  const m = matrix.length;
  const n = matrix[0].length;

  for (let i = 0; i < m; i++) {
    for (let j = 1; j < n; j++) {
      // 原地存储每行的前缀和
      matrix[i][j] += matrix[i][j - 1];
    }
  }
  this.matrix = matrix;
};

/**
 * @param {number} row1
 * @param {number} col1
 * @param {number} row2
 * @param {number} col2
 * @return {number}
 */
NumMatrix.prototype.sumRegion = function (row1, col1, row2, col2) {
  let sum = 0;

  for (let i = row1; i <= row2; i++) {
    sum +=
      col1 === 0
        ? this.matrix[i][col2]
        : this.matrix[i][col2] - this.matrix[i][col1 - 1];
  }

  return sum;
};

/**
 * Your NumMatrix object will be instantiated and called as such:
 * var obj = new NumMatrix(matrix)
 * var param_1 = obj.sumRegion(row1,col1,row2,col2)
 */