<String> 179 ，6， 168

179. Largest Number

冒泡排序，每一轮都把最小的数字选出放在最后。

class Solution {
    public String largestNumber(int[] nums) {
        for(int i = 0; i < nums.length; i++){
            for(int j = 0; j < nums.length - i - 1; j++){
                String s1 = nums[j] + "" + nums[j + 1];
                String s2 = nums[j + 1] + "" + nums[j];
                if(s1.compareTo(s2) < 0){
                    int temp = nums[j];
                    nums[j] = nums[j + 1];
                    nums[j + 1] = temp;
                }
            }
        }
        String res = "";
        for(int i = 0; i < nums.length; i++){
            res += nums[i];
        }
        if(res.charAt(0) == '0'){
            return "0";
        }
        return res;
    }
}

 第二种，重写compare

public class Solution {
     public String largestNumber(int[] num) {
        if(num == null || num.length == 0)
            return "";
        
        // Convert int array to String array, so we can sort later on
        String[] s_num = new String[num.length];
        for(int i = 0; i < num.length; i++)
            s_num[i] = String.valueOf(num[i]);
            
        // Comparator to decide which string should come first in concatenation
        Comparator<String> comp = new Comparator<String>(){
            @Override
            public int compare(String str1, String str2){
                String s1 = str1 + str2;
            String s2 = str2 + str1;
            return s2.compareTo(s1); // reverse order here, so we can do append() later
            }
            };
        
        Arrays.sort(s_num, comp);
                // An extreme edge case by lc, say you have only a bunch of 0 in your int array
                if(s_num[0].charAt(0) == '0')
                    return "0";
            
        StringBuilder sb = new StringBuilder();
        for(String s: s_num)
                sb.append(s);
        
        return sb.toString();
        
    }
}

 6. ZigZag Conversion

之字形走法，先用一个for添加从顶到底的char，第二个for添加斜向上的元素（不包括第一行sb[ 0 ]，故用 i >= 1表示），依次类推。最后将所有sb [ i ] 依次添加到sb[ 0 ]的后面。记得 sb.toString( )。

class Solution {
    public String convert(String s, int numRows) {
     char[] c = s.toCharArray();
        int len = c.length;
        StringBuilder[] sb = new StringBuilder[numRows];
        for(int i = 0; i < numRows; i++){
            sb[i] = new StringBuilder();
        }
        
        int idx = 0;
        while(idx < len){
            for(int i = 0; i < numRows && idx < len; i++){
                sb[i].append(c[idx++]);
            }
            for(int i = numRows - 2; i >= 1 && idx < len; i--){
                sb[i].append(c[idx++]);
            }
        }
        for(int i = 1; i < sb.length; i++){
            sb[0].append(sb[i]);
        }
        return sb[0].toString();
    }
}

168. Excel Sheet Column Title

相当于转化为26进制。先减1 % 26取最小位。再减去余数继续除以26计算更高位。

class Solution {
    public String convertToTitle(int n) {
        StringBuilder sb = new StringBuilder();
        int reminder = 0;
        while(n > 0){
            reminder = (n - 1) % 26;
            sb.insert(0, (char)(reminder + 'A'));
            n = (n - reminder) / 26;
        }
        return sb.toString();
    }
}