README_EN.md

341. Flatten Nested List Iterator

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Description

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

• NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
• int next() Returns the next integer in the nested list.
• boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Your code will be tested with the following pseudocode:

initialize iterator with nestedList
res = []
while iterator.hasNext()
    append iterator.next() to the end of res
return res

If res matches the expected flattened list, then your code will be judged as correct.

 

Example 1:

Input: nestedList = [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:

Input: nestedList = [1,[4,[6]]]
Output: [1,4,6]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

 

Constraints:

• 1 <= nestedList.length <= 500
• The values of the integers in the nested list is in the range [-106, 106].

Solutions

Python3

# """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
#class NestedInteger:
#    def isInteger(self) -> bool:
#        """
#        @return True if this NestedInteger holds a single integer, rather than a nested list.
#        """
#
#    def getInteger(self) -> int:
#        """
#        @return the single integer that this NestedInteger holds, if it holds a single integer
#        Return None if this NestedInteger holds a nested list
#        """
#
#    def getList(self) -> [NestedInteger]:
#        """
#        @return the nested list that this NestedInteger holds, if it holds a nested list
#        Return None if this NestedInteger holds a single integer
#        """

class NestedIterator:
    def __init__(self, nestedList: [NestedInteger]):
        def dfs(nestedList):
            for e in nestedList:
                if e.isInteger():
                    self.vals.append(e.getInteger())
                else:
                    dfs(e.getList())

        self.vals = []
        dfs(nestedList)
        self.cur = 0

    def next(self) -> int:
        res = self.vals[self.cur]
        self.cur += 1
        return res

    def hasNext(self) -> bool:
         return self.cur < len(self.vals)

# Your NestedIterator object will be instantiated and called as such:
# i, v = NestedIterator(nestedList), []
# while i.hasNext(): v.append(i.next())

Java

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *
 *     // @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();
 *
 *     // @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 NestedIterator implements Iterator<Integer> {

    private List<Integer> vals;

    private Iterator<Integer> cur;

    public NestedIterator(List<NestedInteger> nestedList) {
        vals = new ArrayList<>();
        dfs(nestedList);
        cur = vals.iterator();
    }

    @Override
    public Integer next() {
        return cur.next();
    }

    @Override
    public boolean hasNext() {
        return cur.hasNext();
    }

    private void dfs(List<NestedInteger> nestedList) {
        for (NestedInteger e : nestedList) {
            if (e.isInteger()) {
                vals.add(e.getInteger());
            } else {
                dfs(e.getList());
            }
        }
    }
}

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */

TypeScript

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *     If value is provided, then it holds a single integer
 *     Otherwise it holds an empty nested list
 *     constructor(value?: number) {
 *         ...
 *     };
 *
 *     Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     isInteger(): boolean {
 *         ...
 *     };
 *
 *     Return the single integer that this NestedInteger holds, if it holds a single integer
 *     Return null if this NestedInteger holds a nested list
 *     getInteger(): number | null {
 *         ...
 *     };
 *
 *     Set this NestedInteger to hold a single integer equal to value.
 *     setInteger(value: number) {
 *         ...
 *     };
 *
 *     Set this NestedInteger to hold a nested list and adds a nested integer elem to it.
 *     add(elem: NestedInteger) {
 *         ...
 *     };
 *
 *     Return the nested list that this NestedInteger holds,
 *     or an empty list if this NestedInteger holds a single integer
 *     getList(): NestedInteger[] {
 *         ...
 *     };
 * };
 */

class NestedIterator {
    private vals: number[];
    private index: number;

    constructor(nestedList: NestedInteger[]) {
        this.index = 0;
        this.vals = [];
        this.dfs(nestedList);
    }

    dfs(nestedList: NestedInteger[]) {
        for (const v of nestedList) {
            if (v.isInteger()) {
                this.vals.push(v.getInteger());
            } else {
                this.dfs(v.getList());
            }
        }
    }

    hasNext(): boolean {
        return this.index < this.vals.length;
    }

    next(): number {
        return this.vals[this.index++];
    }
}

/**
 * Your ParkingSystem object will be instantiated and called as such:
 * var obj = new NestedIterator(nestedList)
 * var a: number[] = []
 * while (obj.hasNext()) a.push(obj.next());
 */

Rust

// #[derive(Debug, PartialEq, Eq)]
// pub enum NestedInteger {
//   Int(i32),
//   List(Vec<NestedInteger>)
// }
struct NestedIterator {
    index: usize,
    vals: Vec<i32>,
}


/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl NestedIterator {

    fn dfs(nestedList: &Vec<NestedInteger>, vals: &mut Vec<i32>) {
        for ele in nestedList.iter() {
            match ele {
                NestedInteger::Int(val) => vals.push(*val),
                NestedInteger::List(list) => Self::dfs(list, vals),
            }
        }
    }

    fn new(nestedList: Vec<NestedInteger>) -> Self {
        let mut vals = vec![];
        Self::dfs(&nestedList, &mut vals);
        Self {
            vals,
            index: 0,
        }
    }

    fn next(&mut self) -> i32 {
        let res = self.vals[self.index];
        self.index += 1;
        res
    }

    fn has_next(&self) -> bool {
        self.index < self.vals.len()
    }
}

/**
 * Your NestedIterator object will be instantiated and called as such:
 * let obj = NestedIterator::new(nestedList);
 * let ret_1: i32 = obj.next();
 * let ret_2: bool = obj.has_next();
 */

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