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341. Flatten Nested List Iterator

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

Solution 1

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# """
# 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(ls):
            for x in ls:
                if x.isInteger():
                    self.nums.append(x.getInteger())
                else:
                    dfs(x.getList())

        self.nums = []
        self.i = -1
        dfs(nestedList)

    def next(self) -> int:
        self.i += 1
        return self.nums[self.i]

    def hasNext(self) -> bool:
        return self.i + 1 < len(self.nums)


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

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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 {
 *
 *     // @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 empty list if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class NestedIterator implements Iterator<Integer> {
    private List<Integer> nums = new ArrayList<>();
    private int i = -1;

    public NestedIterator(List<NestedInteger> nestedList) {
        dfs(nestedList);
    }

    @Override
    public Integer next() {
        return nums.get(++i);
    }

    @Override
    public boolean hasNext() {
        return i + 1 < nums.size();
    }

    private void dfs(List<NestedInteger> ls) {
        for (var x : ls) {
            if (x.isInteger()) {
                nums.add(x.getInteger());
            } else {
                dfs(x.getList());
            }
        }
    }
}

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

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/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */

class NestedIterator {
public:
    NestedIterator(vector<NestedInteger>& nestedList) {
        auto dfs = [&](auto&& dfs, vector<NestedInteger>& ls) -> void {
            for (auto& x : ls) {
                if (x.isInteger()) {
                    nums.push_back(x.getInteger());
                } else {
                    dfs(dfs, x.getList());
                }
            }
        };
        dfs(dfs, nestedList);
    }

    int next() {
        return nums[++i];
    }

    bool hasNext() {
        return i + 1 < nums.size();
    }

private:
    vector<int> nums;
    int i = -1;
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */

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/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * type NestedInteger struct {
 * }
 *
 * // Return true if this NestedInteger holds a single integer, rather than a nested list.
 * func (this NestedInteger) IsInteger() bool {}
 *
 * // Return the single integer that this NestedInteger holds, if it holds a single integer
 * // The result is undefined if this NestedInteger holds a nested list
 * // So before calling this method, you should have a check
 * func (this NestedInteger) GetInteger() int {}
 *
 * // Set this NestedInteger to hold a single integer.
 * func (n *NestedInteger) SetInteger(value int) {}
 *
 * // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 * func (this *NestedInteger) Add(elem NestedInteger) {}
 *
 * // Return the nested list that this NestedInteger holds, if it holds a nested list
 * // The list length is zero if this NestedInteger holds a single integer
 * // You can access NestedInteger's List element directly if you want to modify it
 * func (this NestedInteger) GetList() []*NestedInteger {}
 */

type NestedIterator struct {
    nums []int
    i    int
}

func Constructor(nestedList []*NestedInteger) *NestedIterator {
    var dfs func([]*NestedInteger)
    nums := []int{}
    i := -1
    dfs = func(ls []*NestedInteger) {
        for _, x := range ls {
            if x.IsInteger() {
                nums = append(nums, x.GetInteger())
            } else {
                dfs(x.GetList())
            }
        }
    }
    dfs(nestedList)
    return &NestedIterator{nums, i}
}

func (this *NestedIterator) Next() int {
    this.i++
    return this.nums[this.i]
}

func (this *NestedIterator) HasNext() bool {
    return this.i+1 < len(this.nums)
}

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/**
 * // 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 nums: number[] = [];
    private i = -1;
    constructor(nestedList: NestedInteger[]) {
        const dfs = (ls: NestedInteger[]) => {
            for (const x of ls) {
                if (x.isInteger()) {
                    this.nums.push(x.getInteger());
                } else {
                    dfs(x.getList());
                }
            }
        };
        dfs(nestedList);
    }

    hasNext(): boolean {
        return this.i + 1 < this.nums.length;
    }

    next(): number {
        return this.nums[++this.i];
    }
}

/**
 * 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());
 */

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

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl NestedIterator {
    fn new(nested_list: Vec<NestedInteger>) -> Self {
        let mut nums = Vec::new();
        Self::dfs(&nested_list, &mut nums);
        NestedIterator { nums, i: 0 }
    }

    fn next(&mut self) -> i32 {
        let result = self.nums[self.i];
        self.i += 1;
        result
    }

    fn has_next(&self) -> bool {
        self.i < self.nums.len()
    }

    fn dfs(nested_list: &Vec<NestedInteger>, nums: &mut Vec<i32>) {
        for ni in nested_list {
            match ni {
                NestedInteger::Int(x) => nums.push(*x),
                NestedInteger::List(list) => Self::dfs(list, nums),
            }
        }
    }
}

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