### 题目

Given an encoded string, return it’s decoded string.

The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times. Note that k is guaranteed to be a positive integer.

You may assume that the input string is always valid; No extra white spaces, square brackets are well-formed, etc.

Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k. For example, there won’t be input like 3a or 2[4].

Examples:

s = "3[a]2[bc]", return "aaabcbc".
s = "3[a2[c]]", return "accaccacc".
s = "2[abc]3[cd]ef", return "abcabccdcdcdef".


### 用Stack

#### 代码

class Solution {
public String decodeString(String s) {
Stack<Block> stack = new Stack<Block>();
int cur = 0;
stack.push(new Block(1));
while (cur < s.length()) {
if (Character.isDigit(s.charAt(cur))) {
int end = cur + 1;
while (Character.isDigit(s.charAt(end))) { end++; }
int num = Integer.valueOf(s.substring(cur,end));
stack.push(new Block(num));
cur = end;
} else if (s.charAt(cur) == ']') {
Block curr = stack.pop();
StringBuilder atom = curr.sb;
StringBuilder sub = new StringBuilder();
for (int i = 0; i < curr.repeat; i++) {
sub.append(atom);
}
stack.peek().sb.append(sub);
} else {
stack.peek().sb.append(s.charAt(cur));
}
cur++;
}
return stack.pop().sb.toString();
}
private class Block {
private int repeat;
private StringBuilder sb = new StringBuilder();
private Block(int num) { repeat = num; }
}
}

class Solution {
public String decodeString(String s) {
Stack<Integer> times = new Stack<>();
Stack<String> atoms = new Stack<>();
int cur = 0;
times.push(1);
atoms.push("");
while (cur < s.length()) {
if (Character.isDigit(s.charAt(cur))) {
int end = cur + 1;
while (Character.isDigit(s.charAt(end))) { end++; }
int num = Integer.valueOf(s.substring(cur,end));
times.push(num);
atoms.push("");
cur = end;
} else if (s.charAt(cur) == ']') {
int repeat = times.pop();
String currAtom = atoms.pop();
String sub = "";
for (int i = 0; i < repeat; i++) {
sub += currAtom;
}
atoms.push(atoms.pop() + sub);
} else {
atoms.push(atoms.pop() + s.charAt(cur));
}
cur++;
}
return atoms.pop();
}
}