前序遍历（ABDECF）：

typedef struct TreeNode {
    int data;
    struct TreeNode *left;
    struct TreeNode *right;
    struct TreeNode *parent;
} TreeNode;
 
void pre_order(TreeNode *Node) {
    if(Node != NULL) {
        printf("%d ", Node->data);
        pre_order(Node->left);
        pre_order(Node->right);
    }
}

中序遍历（DBEAFC）：

typedef struct TreeNode {
    int data;
    struct TreeNode *left;
    struct TreeNode *right;
    struct TreeNode *parent;
} TreeNode;
 
void middle_order(TreeNode *Node) {
    if(Node != NULL) {
        middle_order(Node->left);
        printf("%d ", Node->data);
        middle_order(Node->right);
    }
}

后续遍历（DEBFCA）：

typedef struct TreeNode {
    int data;
    struct TreeNode *left;
    struct TreeNode *right;
    struct TreeNode *parent;
} TreeNode;
 
void last_order(TreeNode *Node) {
    if(Node != NULL) {
        last_order(Node->left);
        last_order(Node->right);
        printf("%d ", Node->data);
    }
}

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