6. ZigZag Conversion

StringBuffer类可以append reverse insert deleteCharAt setCharAt

其实直接用String的array也ok

 

 

//New 这个方法挺好 i控制遍历，两个循环分别控制放哪里

int i = 0;
while (i < len) {
            for (int idx = 0; idx < nRows && i < len; idx++) // vertically down
                sb[idx].append(c[i++]);
            for (int idx = nRows-2; idx >= 1 && i < len; idx--) // obliquely up
                sb[idx].append(c[i++]);
        }

//Old
class Solution {
    public String convert(String s, int numRows) {
        if(s == null || s.length() == 1 || s.length() == 0 || s.length() == 2 || numRows == 1) return s;
        char[] str = s.toCharArray();
        HashMap<Integer, String> map = new HashMap<>();
        for(int i = 0; i < str.length; i++) {
            int key = i % (2*numRows - 2);
            if(key > numRows - 1) {
                key = (2*numRows - 2) - key;
            }
            if(!map.containsKey(key)) {
                map.put(key, "");
            }
            map.put(key, map.get(key) + str[i]);
        }
        String res = "";
        for(int i = 0; i < numRows; i++) {
            if(map.get(i) != null) {             //numRows超过string长度的时候 map中可能出现null 所以要判断
                res = res + map.get(i);    
            }
        }
        return res;
    }
}