【leetcode】119. Pascal's Triangle II

构建Pascal三角形，使用O(k)的空间

/** 
 * @author          johnsondu 
 * @problem         Pascal's Triangle II
 * @time            O(nk)
 * @space           O(k) 
 * @url             https://leetcode.com/problems/pascals-triangle-ii/
 * @strategy        construction problems. 
 * @status          Accepted,   beats 11.64% of cpp submissions. 0ms 
 * @time            15:21, Nov 6th 2015 
 * @require         using O(k) space, the following is O(2k)
 */  

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        if(rowIndex < 0) return vector<int> {};
        if(rowIndex == 0) return vector<int> {1};
        if(rowIndex == 1) return vector<int> {1, 1};
        int idx = 2;
        vector<int> vec(rowIndex + 1, 1);
        vector<int> ans(rowIndex + 1, 1);
        for(int i = 2; i <= rowIndex; i ++) {
            vec = ans;
            ans[0] = vec[0];
            for(int j = 1; j < idx; j ++) {
                ans[j] = vec[j] + vec[j-1];
            }
            ans[idx] = 1;
            idx ++;
        }
        return ans;
    }
};

// right after is precisely O(k)
class Solution {
public:
    vector<int> getRow(int rowIndex) {
        vector<int> ans;
        for(int i = 0; i < rowIndex + 1; i ++) {
            ans.push_back(1);
            for(int j = i-1; j > 0; i --)
                ans[j] += ans[j-1];
        }
        return ans;
    }
};