#2732 - Find a Good Subset of the Matrix

Problem Description

You are given a 0-indexed m x n binary matrix grid.

Let us call a non-empty subset of rows good if the sum of each column of the subset is at most half of the length of the subset.

More formally, if the length of the chosen subset of rows is k, then the sum of each column should be at most floor(k / 2).

Return an integer array that contains row indices of a good subset sorted in ascending order.

If there are multiple good subsets, you can return any of them. If there are no good subsets, return an empty array.

A subset of rows of the matrix grid is any matrix that can be obtained by deleting some (possibly none or all) rows from grid.

Solution

/**
 * @param {number[][]} grid
 * @return {number[]}
 */
var goodSubsetofBinaryMatrix = function(grid) {
  const rows = grid.length;
  const cols = grid[0].length;
  const rowMasks = new Map();

  for (let i = 0; i < rows; i++) {
    let mask = 0;
    for (let j = 0; j < cols; j++) {
      mask |= grid[i][j] << j;
    }
    if (mask === 0) return [i];
    rowMasks.set(mask, i);
  }

  for (const [mask1, i] of rowMasks) {
    for (const [mask2, j] of rowMasks) {
      if ((mask1 & mask2) === 0) return [Math.min(i, j), Math.max(i, j)];
    }
  }

  return [];
};