Count how many 1s

<aside> 💡 Start with a cell. Check conditions, process call, search around.

</aside>

General approach:

count = 0;

function dfs(cell) {

  // 1. Check conditions
  if (cell === 0 || isOutsideGrid()) {
    return;
  }

  // 2. Process cell
  count++;
  matrix[cell] = 0;

  // 3. Search around
  dfs(right);
  dfs(left);
  dfs(top);
  dfs(bottom);

}

• Solution
• Intuition

Number of Islands

<aside> 💡 Run DFS on every cell of the grid which is “1”. After DFS, increase count and continue looping.

</aside>

https://leetcode.com/problems/number-of-islands/

function dfs(row, col) {

  // 1. Check conditions
  if (
    row < 0 ||
    col < 0 ||
    row >= grid.length ||
    col >= grid[0].length ||
    grid[row][col] === "0"
  ) {
    return;
  }

  // 2. Process cell
  grid[row][col] = "0";

  // 3. Search around
  dfs(row, col + 1);
  dfs(row, col - 1);
  dfs(row + 1, col);
  dfs(row - 1, col);

}

• Solution