[LeetCode][JavaScript]Pascal's Triangle II

Pascal's Triangle II

Given an index k, return the kth row of the Pascal's triangle.

For example, given k = 3,
Return [1,3,3,1].

Note:
Could you optimize your algorithm to use only O(k) extra space?

https://leetcode.com/problems/pascals-triangle-ii/

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要求常数级的空间复杂度。

只维护一行的数据，每次从后往前算（因为每次是当前的值加上前一个的值，从后往前就不会影响结果），最后补上1。

/**
 * @param {number} rowIndex
 * @return {number[]}
 */
var getRow = function(rowIndex) {
    if(rowIndex === 0){
        return [1];
    }else if(rowIndex === 1){
        return [1, 1];
    }

    var n = rowIndex - 1;
    var row = [1, 1];
    while(n--){
        for(var i = row.length - 1; i >= 1; i--){
            row[i] = row[i] + row[i - 1];
        }
        row.push(1);
    }
    return row;
};