[LeetCode] 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:
  • String is non-empty.
  • String does not contain white spaces.
  • 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.

Thought process:
Iterate through the string. There are five cases:
  1. Digit: keep track of current number.
  2. [: push the current NestedInteger onto stack and create a new NestedInteger.
  3. -: set sign to -1.
  4. ,: if the previous character is a digit, add the number into current NestedInteger. If the previous character is ], add the current NestedInteger to previous NestedInteger.
  5. ]: current NestedInteger is finished. Pop the NestedInteger from top of the stack.

Solution:

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/**
 * // 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.length() == 0) {
            return new NestedInteger();
        }
        if (s.charAt(0) != '[') {
            return new NestedInteger(Integer.parseInt(s));
        }
        NestedInteger current = null;
        Stack<NestedInteger> stack = new Stack<>();
        int left = 0;
        int right = 0;
        
        for (; right < s.length(); right++) {
            char c = s.charAt(right);
            if (c == '[') {
                if (current != null) {
                    stack.push(current);
                }
                current = new NestedInteger();
                left = right + 1;
            } else if (c == ']') {
                String number = s.substring(left, right);
                if (number.length() > 0) {
                    current.add(new NestedInteger(Integer.parseInt(number)));
                }
                if (!stack.isEmpty()) {
                    NestedInteger parent = stack.pop();
                    parent.add(current);
                    current = parent;
                }
                left = right + 1;
            } else if (c == ',') {
                if (s.charAt(right - 1) != ']') {
                    current.add(new NestedInteger(Integer.parseInt(s.substring(left, right))));
                }
                left = right + 1;
            }
        }
        
        return current;
    }
}

Time complexity: O(n).
Location: San Jose, CA, USA

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