LeetCode //C - 341. Flatten Nested List Iterator

341. Flatten Nested List Iterator

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 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 [ − 1 0 6 , 1 0 6 ] [-10^6, 10^6] [−106,106].

From: LeetCode

Link: 341. Flatten Nested List Iterator

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Solution:

Ideas:

1. resizeFlattenedList Function: This function checks if the flattenedList array needs more space (i.e., when size >= capacity). If it does, the capacity is doubled, and the array is reallocated to the new size.

2. Dynamic Array Allocation: The flattenedList array starts with an initial capacity of 100. Whenever the array needs more space, it doubles its capacity. This ensures that the array can grow as needed without causing a buffer overflow.

3. Flattening Logic: The flatten function remains responsible for traversing the nested list structure. It now calls resizeFlattenedList before adding each integer to ensure there's enough space in the flattenedList array.

Code:

/**
 * *********************************************************************
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * *********************************************************************
 *
 * // Return true if this NestedInteger holds a single integer, rather than a nested list.
 * bool NestedIntegerIsInteger(struct NestedInteger *);
 *
 * // 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 NestedIntegerGetInteger(struct NestedInteger *);
 *
 * // 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
 * struct NestedInteger **NestedIntegerGetList(struct NestedInteger *);
 *
 * // Return the nested list's size that this NestedInteger holds, if it holds a nested list
 * // The result is undefined if this NestedInteger holds a single integer
 * int NestedIntegerGetListSize(struct NestedInteger *);
 * };
 */
// This structure will hold our iterator state
struct NestedIterator {
    int *flattenedList;  // Array to hold the flattened list
    int size;            // Size of the flattened list
    int capacity;        // Capacity of the flattened list
    int index;           // Current index in the flattened list
};

// Helper function to resize the flattened list array if needed
void resizeFlattenedList(struct NestedIterator *iter) {
    if (iter->size >= iter->capacity) {
        iter->capacity *= 2;  // Double the capacity
        iter->flattenedList = (int *)realloc(iter->flattenedList, iter->capacity * sizeof(int));
    }
}

// Helper function to flatten the nested list recursively
void flatten(struct NestedInteger **nestedList, int nestedListSize, struct NestedIterator *iter) {
    for (int i = 0; i < nestedListSize; i++) {
        if (NestedIntegerIsInteger(nestedList[i])) {
            // If it's an integer, add it to the result array
            resizeFlattenedList(iter);
            iter->flattenedList[iter->size++] = NestedIntegerGetInteger(nestedList[i]);
        } else {
            // If it's a list, recursively flatten it
            struct NestedInteger **subList = NestedIntegerGetList(nestedList[i]);
            int subListSize = NestedIntegerGetListSize(nestedList[i]);
            flatten(subList, subListSize, iter);
        }
    }
}

// Initializes the iterator with the nested list
struct NestedIterator *nestedIterCreate(struct NestedInteger** nestedList, int nestedListSize) {
    // Allocate memory for the iterator
    struct NestedIterator *iter = (struct NestedIterator *)malloc(sizeof(struct NestedIterator));
    iter->capacity = 100;  // Start with an initial capacity
    iter->flattenedList = (int *)malloc(iter->capacity * sizeof(int));
    iter->size = 0;
    iter->index = 0;

    // Flatten the nested list into the flattenedList array
    flatten(nestedList, nestedListSize, iter);
    
    return iter;
}

// Returns true if there are more integers to return
bool nestedIterHasNext(struct NestedIterator *iter) {
    return iter->index < iter->size;
}

// Returns the next integer in the flattened list
int nestedIterNext(struct NestedIterator *iter) {
    return iter->flattenedList[iter->index++];
}

// Deallocates memory previously allocated for the iterator
void nestedIterFree(struct NestedIterator *iter) {
    free(iter->flattenedList);
    free(iter);
}
/**
 * Your NestedIterator will be called like this:
 * struct NestedIterator *i = nestedIterCreate(nestedList, nestedListSize);
 * while (nestedIterHasNext(i)) printf("%d\n", nestedIterNext(i));
 * nestedIterFree(i);
 */