Given an m x n matrix mat, return an array of all the elements of the array in a diagonal order.

findDiagonalOrder.js

/**
 * @param {number[][]} mat
 * @return {number[]}
 */
var findDiagonalOrder = function(mat) {
    // Edge case: empty matrix
    if (!mat || mat.length === 0 || mat[0].length === 0) return [];

    const m = mat.length;       // number of rows
    const n = mat[0].length;    // number of columns
    const result = [];

    let row = 0, col = 0;
    let directionUp = true;     // true = moving up-right, false = down-left

    // We need to collect m * n elements
    while (result.length < m * n) {
        result.push(mat[row][col]);

        if (directionUp) {
            // Moving up-right
            if (col === n - 1) {
                // Hit right boundary, move down
                row++;
                directionUp = false;
            } else if (row === 0) {
                // Hit top boundary, move right
                col++;
                directionUp = false;
            } else {
                // Normal up-right move
                row--;
                col++;
            }
        } else {
            // Moving down-left
            if (row === m - 1) {
                // Hit bottom boundary, move right
                col++;
                directionUp = true;
            } else if (col === 0) {
                // Hit left boundary, move down
                row++;
                directionUp = true;
            } else {
                // Normal down-left move
                row++;
                col--;
            }
        }
    }

    return result;
};