LeetCode Mini Parser

原题链接在这里：https://leetcode.com/problems/mini-parser/description/

题目：

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.

题解：

遇到'[', 说明要往下走一层. 把current NestedInteger push进stack, 创建个新的.

遇到']', 说明本层走完, 要往上走一层. pop出上层NestedInteger, 吧current NestedInteger 加到 上层NestedInteger里.

遇到",", 如果前面不是"]", 说明是当前层NestedInteger加上个Integer.

l 是一段数字开始的index. r是一段数字结束的index+1.

如果开始就不是"[", 特例说明这就是一个单层的数.

Time Complexity: O(s.length()).

Space: O(s.length()). stack space.

AC Java:

/**
 * // 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();
 * }
 */
class Solution {
    public NestedInteger deserialize(String s) {
        if(s.charAt(0) != '['){
            return new NestedInteger(Integer.valueOf(s));
        }
        
        NestedInteger cur = null;
        Stack<NestedInteger> stk = new Stack<NestedInteger>();
        
        int l = 0;
        for(int r = 0; r<s.length(); r++){
            char c = s.charAt(r);
            if(c == '['){
                if(cur != null){
                    stk.push(cur);
                }
                cur = new NestedInteger();
                l = r+1;
            }else if(c == ']'){
                String num = s.substring(l, r);
                if(num.length() != 0){
                    cur.add(new NestedInteger(Integer.valueOf(num)));
                }
                if(!stk.isEmpty()){
                    NestedInteger top = stk.pop();
                    top.add(cur);
                    cur = top;
                }
                l = r+1;
            }else if(c == ','){
                if(s.charAt(r-1) != ']'){
                    String num = s.substring(l, r);
                    if(num.length() != 0){
                        cur.add(new NestedInteger(Integer.valueOf(num)));
                    }
                }
                l = r+1;
            }
        }
        
        return cur;
    }
}

类似Flatten Nested List Iterator.