118. Pascal’s Triangle (javascript)

Given an integer numRows, return the first numRows of Pascal’s triangle.

In Pascal’s triangle, each number is the sum of the two numbers directly above it as shown:

Example 1:

Input: numRows = 5
Output: [[1],[1,1],[1,2,1],[1,3,3,1],[1,4,6,4,1]]

Example 2:

Input: numRows = 1
Output: [[1]]

Constraints:

• 1 <= numRows <= 30

Solution:

/**
 * @param {number} numRows
 * @return {number[][]}
 */
var generate = function(numRows) {
    let res = [[1]];
    
    while(numRows > 1) {
        let previous = res[res.length - 1];
        // The first row element is always 1.
        let cur = [1];
        for (let i = 1; i < previous.length; i++) {
            // generate: sum of the elements above-and-to-the-left and above-and-to-the-right.
            cur.push(previous[i - 1] + previous[i]);
        }
        // The last row element is always 1.
        cur.push(1);
        res.push(cur.concat());
        numRows--;
    }
    return res;
};