# Short Java solution - Easy to understand - Recursion - Less Code and Full of Explanation

• So basically, we first check if the first char is '[' or not, if it is, we know it is def a list, if it is NOT, then we know it is an integer. (Assume the input is valid).
So if it is a list, we will create a NestedInteger, and add all its "items" one by one to this NestedInteger (of cuz, for each item, we recursively create NestedInteger and return), and finally return this NestedInteger.

``````public NestedInteger deserialize(String s) {
if(s == null || s.isEmpty()) return null;
if(s.charAt(0) != '[') {
///This is a number!! - Easy
return new NestedInteger(Integer.parseInt(s));
}

//this is a list
NestedInteger res = new NestedInteger();
//s = "[aaa, [bbb, ccc], dd]"
s = s.substring(1, s.length()-1); //strip outter '[' and ']'
//Stack is used to keep track of which ',' is the one we need to parse for this level.
//For example, the ',' after 'aaa', once we hit that, we know 'aaa' is an item, and we recursively deserialize 'aaa', but for '[bbb,ccc]' I want this as a whole part, cuz it as a whole is a NestedInteger which I wanna do a recursive call on,
//so once I met the ',' after 'bbb', I am not gonna do anything, I will just keep appending it to a string and later when I hit ',' after 'ccc', I am gonna send the whole '[bbb,ccc]' to the recursive call.
Stack<Character> stack = new Stack<>();
String prev = "";    //keep track of the item in THIS level before ','

// right now s: "aaa,[bbb,ccc],dd". cuz we strip the outter [ ]
for(int i=0; i<s.length(); i++) {
char c = s.charAt(i);
if(stack.isEmpty() && c == ',') {
//if the stack is empty and then we hit ',', we know prev right now is a item for this level.
//If the stack is not empty, that means I am in inside parsing one of the item in this level, I haven't done yet. So I will wait to do recursive call.
prev="";
} else if(c == '['){
stack.push(c);   //This is telling I am inside parsing one item in this level
prev += c;
} else if(c == ']') {
stack.pop();     //def pop out here cuz later when we hit ',' after 'ccc', the stack should be empty, and we will do a recursive call on '[bbb,ccc]'
prev += c;
} else if(c>= '0' && c<='9' || c==',' || c=='-') {
prev += c;
}
}

res.add(deserialize(prev));   //don't forget the last item, cuz we don't have a ',' after the last item in the s

return res;
}``````

• It might not be as efficient as other iterative solution, since it scans the same substring several times, but I think the code is def more readable and easy to understand. Cuz recursion def helps us to make things easier.

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