394. 字串解碼

class Solution {
    public String decodeString(String s) {
        StringBuilder res = new StringBuilder();
        int multi = 0;
        LinkedList<Integer> stack_multi = new LinkedList<>();
        LinkedList<String> stack_res = new LinkedList<>();
        for(Character c : s.toCharArray()) {
            if(c == '[') {
                stack_multi.addLast(multi);
                stack_res.addLast(res.toString());
                multi = 0;
                res = new StringBuilder();
            } else if(c == ']') {
                StringBuilder tmp = new StringBuilder();
                int cur_multi = stack_multi.removeLast();
                for(int i = 0; i < cur_multi; i++) tmp.append(res);
                res = new StringBuilder(stack_res.removeLast() + tmp);
            } else if(c >= '0' && c <= '9') 
                multi = multi * 10 + Integer.parseInt(c + "");
            else res.append(c);
        }
        return res.toString();
    }
}

class Solution {
    public String decodeString(String s) {
        return dfs(s, 0)[0];
    }
    private String[] dfs(String s, int i) {
        StringBuilder res = new StringBuilder();
        int multi = 0;
        while(i < s.length()) {
            if(s.charAt(i) >= '0' && s.charAt(i) <= '9') 
                multi = multi * 10 + Integer.parseInt(String.valueOf(s.charAt(i))); 
            else if(s.charAt(i) == '[') {
                String[] tmp = dfs(s, i + 1);
                i = Integer.parseInt(tmp[0]);
                while(multi > 0) {
                    res.append(tmp[1]);
                    multi--;
                }
            }
            else if(s.charAt(i) == ']') 
                return new String[] { String.valueOf(i), res.toString() };
            else 
                res.append(String.valueOf(s.charAt(i)));
            i++;
        }
        return new String[] { res.toString() };
    } 
}

面試題 05.06. 整數轉換

class Solution {
    public int convertInteger(int A, int B) {
        int c = A^B;
        int count = 0;
        while(c!=0){
            if((c&1)==1){
                count++;
            }
            c >>>= 1;
        }
        return count;
    }
}

// n&(n - 1)可以去掉一個數的二進制表示的最右邊的1
class Solution {
    public int convertInteger(int A, int B) {
        int num = A^B;
        int count = 0;
        while (num != 0) {
            num &= num - 1;
            count++;
        }
        return count;
    }
}

面試題 05.03. 翻轉數位

class Solution {
    public int reverseBits(int num) {
        int maxLen = 0, preLen = 0, curLen = 0, bits = 32;
        while (bits-- > 0) {
            if ((num & 1) == 0) {
                curLen -= preLen;
                preLen = curLen + 1;
            }
            curLen++;
            maxLen = Math.max(maxLen, curLen);
            num >>= 1;
        }
        return maxLen;
    }
}

你知道的越多，你不知道的越多。