118.杨辉三角

/**
 * @param {number} numRows
 * @return {number[][]}
 */
var generate = function (numRows) {
    var res = new Array(numRows)
    for (var i = 0; i < numRows; i++) {
        if (i === 0) {
            res[i] = [1]
            continue
        }
        if (i === 1) {
            res[i] = [1, 1]
            continue
        }
        res[i] = new Array(i + 1)
        for (var j = 0; j < res[i].length; j++) {
            // 第一项和最后一项都是1
            if (j === 0 || j === res[i].length - 1) {
                res[i][j] = 1
                continue
            }
            // res[2][1] = res[1][0] + res[1][1]
            // res[3][1] = res[2][0] + res[2][1]
            // res[3][2] = res[2][1] + res[2][2]
            res[i][j] = res[i - 1][j - 1] + res[i - 1][j]
        }
    }
    console.log(res);
    return res
};

// 输入: numRows = 5
// 输出: [[1],[1,1],[1,2,1],[1,3,3,1],[1,4,6,4,1]]
console.log(generate(5));
console.log(generate(1));