LeetCode 6. Z 字形变换（找规律）

L   C   I   R
E T O E S I I G
E   D   H   N

string convert(string s, int numRows);

示例 1:
输入: s = "LEETCODEISHIRING", numRows = 3
输出: "LCIRETOESIIGEDHN"

示例 2:
输入: s = "LEETCODEISHIRING", numRows = 4
输出: "LDREOEIIECIHNTSG"
解释:
L     D     R
E   O E   I I
E C   I H   N
T     S     G

0： g ---g----g
1：g-2---2---g-2----2
2：g-4---4---g-4----4
.
.
n-2:2---g-2---2---g-2
n-1: g ---g----g

class Solution {
public:
    string convert(string s, int numRows) {
        if(numRows == 1)
            return s;
        int gap = 2*numRows-2;
        int startGap, i, j, k = 0;
        string ans(s.size(),'0');
        for(i = 0; i < numRows; i++) 
        {	//遍历每行
            ans[k++] = s[i];//首字符
            startGap = gap-2*i;//起始间距
            if(startGap==0)
                startGap = gap;
            for(j = i+startGap; j < s.size(); j += startGap)
            {
                ans[k++] = s[j];
                startGap = gap-startGap;//间距交替变化
                if(startGap==0)
                    startGap = gap;
            }
        }
        return ans;
    }
};