P A H N
A P L S I I G
Y I R
And then read line by line: "PAHNAPLSIIGYIR"
Write the code that will take a string and make this conversion given a number of rows:
string convert(string text, int nRows);
convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".
[分析] 找规律类型的题目,给出三种实现,实现1和2均是按找到的规律计算每一行各列元素在原串中的下标,实现3是按照原串的顺序构造出zigzag图,然后从上到下逐行遍历,时间和空间使用上均不如前两种好,但是更直观易懂。
[ref]
http://blog.unieagle.net/2012/11/08/leetcode%E9%A2%98%E7%9B%AE%EF%BC%9Azigzag-conversion/
// method 3 public String convert(String s, int numRows) { if (s == null || s.length() <= 1 || numRows == 1) return s; // build zigzag map ArrayList<ArrayList<Character>> map = new ArrayList<ArrayList<Character>>(numRows); for (int i = 0; i < numRows; i++) map.add(new ArrayList<Character>()); boolean down = true; int n = s.length(); int i = 0, row = 0; while (i < n) { if (row < numRows && down) { map.get(row++).add(s.charAt(i++)); } else if (row >= 0 && !down) { map.get(row--).add(s.charAt(i++)); } else {//ajust direction row += down ? -2 : 2; down = !down; } } // read map row by row StringBuilder ret = new StringBuilder(); for (ArrayList<Character> line : map) { for (Character c : line) { ret.append(c); } } return ret.toString(); } // method 2 public String convert2(String s, int numRows) { if (s == null || s.length() <= 1 || numRows == 1) return s; int n = s.length(); int delta = 2 * numRows - 2, diff1 = 0; StringBuilder ret = new StringBuilder(); for (int i = 0; i < numRows; i++) { int j = i; diff1 = delta - 2 * i; while (j < n) { if (diff1 > 0) ret.append(s.charAt(j)); j += diff1; diff1 = delta - diff1; } } return ret.toString(); } // method 1 public String convert1(String s, int numRows) { if (s == null || s.length() <= 1 || numRows == 1) return s; int n = s.length(); if (n < numRows) numRows = n; int delta = 2 * numRows - 2; StringBuilder ret = new StringBuilder(); for (int i = 0; i < numRows; i++) { int j = i; while (j < n) { ret.append(s.charAt(j)); int next = j + delta - 2 * i; if (i != 0 && i != numRows - 1 && next < n) { ret.append(s.charAt(next)); } j += delta; } } return ret.toString(); }