385. Mini Parser

Given a nested list of integers represented as a string, implement a parser to deserialize it.

Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Note: You may assume that the string is well-formed:

1. String is non-empty.
2. String does not contain white spaces.
3. String contains only digits 0-9, [, - ,, ].

Example 1:

Given s = "324",

You should return a NestedInteger object which contains a single integer 324.

Example 2:

Given s = "[123,[456,[789]]]",

Return a NestedInteger object containing a nested list with 2 elements:

1. An integer containing value 123.
2. A nested list containing two elements:
    i.  An integer containing value 456.
    ii. A nested list with one element:
         a. An integer containing value 789.

解题思路为采用stack。

1. If encounters '[', start a new NestedInteger and add it to current NestedInteger, then push it to stack.
2. If encounters ']', end current NestedInteger and pop a NestedInteger from stack to continue.
3. If encounters ',', append a new number to current NestedInteger, if this comma is not right after a brackets.

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *     // Constructor initializes an empty nested list.
 *     public NestedInteger();
 *
 *     // Constructor initializes a single integer.
 *     public NestedInteger(int value);
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     public Integer getInteger();
 *
 *     // Set this NestedInteger to hold a single integer.
 *     public void setInteger(int value);
 *
 *     // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 *     public void add(NestedInteger ni);
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return null if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class Solution {
    public NestedInteger deserialize(String s) {
        if(!s.startsWith("[")) return new NestedInteger(Integer.valueOf(s));
        Stack<NestedInteger> stack = new Stack<>();
        NestedInteger res = new NestedInteger();
        stack.push(res);
        int start = 1;
        for(int i = 1; i< s.length(); i++) {
            char c = s.charAt(i);
            if(c == '[') {
                NestedInteger ni = new NestedInteger();
                stack.peek().add(ni);
                stack.push(ni);
                start = i + 1;
            } else if(c == ',' || c == ']') {
                if(i > start) {
                    Integer val = Integer.valueOf(s.substring(start, i));
                    stack.peek().add(new NestedInteger(val));
                }
                start = i + 1;
                if(c == ']') {
                    stack.pop();
                }
            }
        }
        return res;
    }
}