leetcode 94.二叉树的中序遍历（binary tree inorder traversal）C语言

1.description

https://leetcode-cn.com/problems/binary-tree-inorder-traversal/

给定一个二叉树，返回它的中序 遍历。

示例:

输入: [1,null,2,3]

输出: [1,3,2]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

2.solution

2.1 递归

int get_size(struct TreeNode *root){
    if(root == NULL) return 0;

    return get_size(root->left) + get_size(root->right) + 1;
}

void inorder(int *returnSize, int *res, struct TreeNode *root){
    if(root == NULL) return;
    
    inorder(returnSize, res, root->left);
    res[(*returnSize)++] = root->val;
    inorder(returnSize, res, root->right);
}

int* inorderTraversal(struct TreeNode* root, int* returnSize){
    *returnSize = 0;
    int node_size = get_size(root);
    int *res = (int*)malloc(sizeof(int)*node_size);
    inorder(returnSize, res, root);

    return res;
}

2.2 迭代

#define MAX_LEN 10000

int get_size(struct TreeNode *root){
    if(root == NULL) return 0;

    return get_size(root->left) + get_size(root->right) + 1;
}

int* inorderTraversal(struct TreeNode* root, int* returnSize){
    *returnSize = 0;
    if(root==NULL) return NULL;

    int node_size = get_size(root);
    int *res = (int*)malloc(sizeof(int)*node_size);
    struct TreeNode *stack[MAX_LEN];
    struct TreeNode *p = NULL;
    int top = -1;
    stack[++top] = root;

    while(top!=-1){
        while(stack[top]!=NULL){
            stack[top+1] = stack[top]->left;
            top++;
        }
        top--; // 空指针出栈
        if(top!=-1){
            p = stack[top];
            res[(*returnSize)++] = p->val;
            stack[top] = p->right;
        }
    }

    return res;
}