Question

Formatted question description: https://leetcode.ca/all/385.html

 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.

 @tag-stack

Algorithm

First judge whether s is empty, and return directly if it is empty.

If it is not empty, see if the first character is [,

  • If it is not, s is an integer, and we return the result directly.
  • If the first character is [,
    • And the length of s is less than or equal to 2, indicating that there is no content, and the result is returned directly.
    • On the other hand, if the length of s is greater than 2, we start traversing from i=1, we need a variable start to record the start position of a certain layer, and use cnt to record whether the actual position is the same depth, cnt=0 means Same depth.

Since each paragraph in the middle is separated by a comma, when we judge that cnt is 0, and the current character is a comma or has reached the end of the string, we take out the string between start and the current position to call the function recursively, Add the returned result to res, and then update start to i+1.

  • If you encounter [, the counter cnt increments by 1,
  • If encountering ], the counter cnt will decrement by 1

Code

Java

import java.util.List;
import java.util.Stack;

public class Mini_Parser {

    class Solution_recursion {
        public NestedInteger deserialize(String s) {
            if (s.isEmpty()) return new NestedInteger();
            if (s.charAt(0) != '[') return new NestedInteger(Integer.valueOf(s));
            if (s.length() <= 2) return new NestedInteger(); // for cases, [[]] => []

            NestedInteger res = new NestedInteger();
            int start = 1; // 0 is `[`, skip it
            int bracketCount = 0;

            for (int i = 1; i < s.length(); ++i) {
                if (bracketCount == 0 && (s.charAt(i) == ',' || i == s.length() - 1)) {
                    res.add(deserialize(s.substring(start, i)));
                    start = i + 1;
                } 
                else if (s.charAt(i) == '[') ++bracketCount;
                else if (s.charAt(i) == ']') --bracketCount;
            }
            return res;
        }
    }

    class Solution {
        public NestedInteger deserialize(String s) {
            if (s.isEmpty()) {
                return null;
            }

            if (s.charAt(0) != '[') {// ERROR: special case
                return new NestedInteger(Integer.valueOf(s));
            }

            Stack<NestedInteger> stack = new Stack<>();
            NestedInteger curr = null;
            int l = 0; // l shall point to the start of a number substring;
            // r shall point to the end+1 of a number substring
            for (int r = 0; r < s.length(); r++) {
                char ch = s.charAt(r);
                if (ch == '[') {
                    if (curr != null) {
                        stack.push(curr);
                    }
                    curr = new NestedInteger();
                    l = r + 1;
                } else if (ch == ']') {
                    String num = s.substring(l, r);
                    if (!num.isEmpty()) { // if empty, then just continue pop from stack
                        curr.add(new NestedInteger(Integer.valueOf(num)));
                    }
                    if (!stack.isEmpty()) {
                        NestedInteger pop = stack.pop();
                        pop.add(curr);
                        curr = pop;
                    }
                    l = r+1;
                } else if (ch == ',') {
                    if (s.charAt(r - 1) != ']') {
                        String num = s.substring(l, r);
                        curr.add(new NestedInteger(Integer.valueOf(num)));
                    }
                    l = r+1;
                }
            }

            return curr;
        }
    }


    // This is the interface that allows for creating nested lists.
    // You should not implement it, or speculate about its implementation
    class 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 false; };

        // @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() { return new Integer(1); }

        // 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() { return null; };
    }
}

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