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.

Constraints

• 1 <= nestedList.length <= 500

• The values of the integers in the nested list is in the range [-106, 106].

Approach

Links

• GeeksforGeeks

• Leetcode

• ProgramCreek

• YouTube

Examples

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].

Solutions

NestedInteger

// 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();
}

Solution 1

/**
 * Time complexity : 
 * Space complexity : 
 */

public class NestedIterator implements Iterator<Integer> {
    private Stack<NestedInteger> stack = new Stack();

    public NestedIterator(List<NestedInteger> nestedList) {
        if(nestedList == null || nestedList.isEmpty()) {
            return;
        }
        for(int i = nestedList.size()-1; i >= 0; i--) {
            stack.push(nestedList.get(i));
        }
    }

    @Override
    public Integer next() {
        return stack.pop().getInteger();
    }

    @Override
    public boolean hasNext() {
        while(!stack.isEmpty()) {
            NestedInteger top = stack.peek();
            if(top.isInteger()) {
                return true;
            } else {
                stack.pop();
                List<NestedInteger> topList = top.getList();
                for(int i = topList.size()-1; i >= 0; i--) {
                    stack.push(topList.get(i));
                }
            }
        }
        return false;
    }
}

/**
 * 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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