二叉树的前、中、后序遍历

解题思路：
递归遍历前序：根、左、右
递归遍历中序：左、根、右
递归遍历后序：左、右、根
要放到数组里面开辟空间，要知道树的大小，然后递归遍历数组下标用指针，因为递归返回时原下标不动，否则下标会继续增加，结果会发生错误。
前序遍历：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */


/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
int SizeTree(struct TreeNode* root)
{
    if(root == NULL)
        return 0;
    return SizeTree(root->left) + SizeTree(root->right) + 1;
}
void _preorderTraversal(struct TreeNode* root, int* a,int* i)
{
    if(root == NULL)
        return;
    
    a[*i] = root->val;
    ++*i;
    _preorderTraversal(root->left,a,i);
    _preorderTraversal(root->right,a,i);
}
int* preorderTraversal(struct TreeNode* root, int* returnSize){
    int size = SizeTree(root);
    int *a = (int*)malloc(sizeof(int)*size);
    int i = 0;
    _preorderTraversal(root,a,&i);
    *returnSize = size;

    return a;
}

中序遍历：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */


/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
 int TreeSize(struct TreeNode* root)
 {
    if(root == NULL)
        return 0;
    return TreeSize(root->left) + TreeSize(root->right) +1;
 }
 void _inorderTraversal(struct TreeNode* root,int* a,int* pi)
 {  
     if(root == NULL)
        return;


     _inorderTraversal(root->left,a,pi);
     a[*pi] = root->val;
     ++*pi;
     _inorderTraversal(root->right,a,pi);
 }
int* inorderTraversal(struct TreeNode* root, int* returnSize){
    int size = TreeSize(root);
    int* a = (int*)malloc(sizeof(int)*size);
    int i = 0;
    _inorderTraversal(root,a,&i);
    *returnSize = size;

    return a;
}

后序遍历：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */


/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
int TreeSize(struct TreeNode* root)
{
    if(root == NULL)
        return 0;
    return TreeSize(root->left)+TreeSize(root->right)+1;
}
void _postorderTraversal(struct TreeNode* root, int* a,int* i){
    if(root == NULL)
        return;
    _postorderTraversal(root->left,a,i);
    _postorderTraversal(root->right,a,i);
    a[*i] = root->val;
    ++*i;
}
int* postorderTraversal(struct TreeNode* root, int* returnSize){
    int size = TreeSize(root);
    int* a = (int*)malloc(sizeof(int)*size);
    int i =0;
    _postorderTraversal(root,a,&i);
    *returnSize = size;

    return a;
}