前序遍历二叉树, 非递归实现

题目

题目来源: LeetCode
前序遍历二叉树, 非递归实现

实现

//先实现栈, 利用栈实现非递归
int* _preorderTraversal(struct TreeNode* root, int* a){
    Stack q;
    StackInit(&q);
    int index = 0;
    //当栈不为空, root为空时
    while(StackEmpty(&q) == 1 || root){
        while(root){
            StackPush(&q, root);
            struct TreeNode* cur = StackTop(&q);
            a[index++] = cur->val;
            root = root->left;
        }
        struct TreeNode* cur = StackTop(&q);
        root = cur->right;
        StackPop(&q);
    }
    return a;
}
//获取二叉树的节点个数
int _returnSize(struct TreeNode* root){
    if(!root){
        return 0;
    }
    return _returnSize(root->left) + _returnSize(root->right) + 1;
}
int* preorderTraversal(struct TreeNode* root, int* returnSize){
    int* a = NULL;
    *returnSize = _returnSize(root);
    //开辟数组
    a = (int*)malloc(sizeof(int) * (*returnSize));
    if(!root){
        return a;
    }
    a = _preorderTraversal(root, a);
    return a;
}