LeetCode //C - 103. Binary Tree Zigzag Level Order Traversal

103. Binary Tree Zigzag Level Order Traversal

Given the root of a binary tree, return the zigzag level order traversal of its nodes’ values. (i.e., from left to right, then right to left for the next level and alternate between).
 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

Constraints:

• The number of nodes in the tree is in the range [0, 2000].
• -100 <= Node.val <= 100

From: LeetCode
Link: 103. Binary Tree Zigzag Level Order Traversal

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Solution:

Ideas：

1. Use a double-ended queue (deque) to keep track of the current level of nodes.
2. Use a variable isLeftToRight to determine the direction of traversal. We start from left-to-right for the first level.
3. For each level:
   • If isLeftToRight is true, pop nodes from the front of the deque and push their left child followed by their right child to the back.
   • Otherwise, pop nodes from the back of the deque and push their right child followed by their left child to the front.
4. Keep track of the number of nodes at the current level to determine when to switch to the next level and reverse the traversal direction.

Code:

typedef struct {
    
    
    struct TreeNode **data;
    int front;
    int rear;
    int size;
    int capacity;
} Deque;

Deque* createDeque(int capacity) {
    
    
    Deque *deque = (Deque *)malloc(sizeof(Deque));
    deque->data = (struct TreeNode **)malloc(capacity * sizeof(struct TreeNode *));
    deque->front = 0;
    deque->rear = -1;
    deque->size = 0;
    deque->capacity = capacity;
    return deque;
}

void pushFront(Deque *deque, struct TreeNode *node) {
    
    
    deque->front = (deque->front - 1 + deque->capacity) % deque->capacity;
    deque->data[deque->front] = node;
    deque->size++;
}

void pushRear(Deque *deque, struct TreeNode *node) {
    
    
    deque->rear = (deque->rear + 1) % deque->capacity;
    deque->data[deque->rear] = node;
    deque->size++;
}

struct TreeNode* popFront(Deque *deque) {
    
    
    struct TreeNode *node = deque->data[deque->front];
    deque->front = (deque->front + 1) % deque->capacity;
    deque->size--;
    return node;
}

struct TreeNode* popRear(Deque *deque) {
    
    
    struct TreeNode *node = deque->data[deque->rear];
    deque->rear = (deque->rear - 1 + deque->capacity) % deque->capacity;
    deque->size--;
    return node;
}

int** zigzagLevelOrder(struct TreeNode* root, int* returnSize, int** returnColumnSizes) {
    
    
    if (!root) {
    
    
        *returnSize = 0;
        return NULL;
    }

    int **result = (int **)malloc(2000 * sizeof(int *));
    *returnColumnSizes = (int *)malloc(2000 * sizeof(int));
    *returnSize = 0;

    Deque *deque = createDeque(2000);
    pushRear(deque, root);
    int isLeftToRight = 1;

    while (deque->size > 0) {
    
    
        int levelSize = deque->size;
        (*returnColumnSizes)[*returnSize] = levelSize;
        result[*returnSize] = (int *)malloc(levelSize * sizeof(int));

        for (int i = 0; i < levelSize; i++) {
    
    
            struct TreeNode *current;
            if (isLeftToRight) {
    
    
                current = popFront(deque);
                result[*returnSize][i] = current->val;
                if (current->left) pushRear(deque, current->left);
                if (current->right) pushRear(deque, current->right);
            } else {
    
    
                current = popRear(deque);
                result[*returnSize][i] = current->val;
                if (current->right) pushFront(deque, current->right);
                if (current->left) pushFront(deque, current->left);
            }
        }

        isLeftToRight = !isLeftToRight;
        (*returnSize)++;
    }

    free(deque->data);
    free(deque);
    return result;
}