c语言用非递归实现二叉树的前序、中序、后序遍历算法。 注意：非递归算法写前序、中序、后序遍历算法

二叉树的前序、中序、后序遍历算法可以使用栈来实现非递归遍历。

以前序遍历为例：

#include <stdio.h>
#include <stdlib.h>

struct TreeNode {
    int val;
    struct TreeNode* left;
    struct TreeNode* right;
};

struct StackNode {
    struct TreeNode* node;
    struct StackNode* next;
};

void push(struct StackNode** stack, struct TreeNode* node) {
    struct StackNode* new_node = (struct StackNode*)malloc(sizeof(struct StackNode));
    new_node->node = node;
    new_node->next = *stack;
    *stack = new_node;
}

struct TreeNode* pop(struct StackNode** stack) {
    if (*stack == NULL) {
        return NULL;
    }
    struct StackNode* top = *stack;
    struct TreeNode* node = top->node;
    *stack = top->next;
    free(top);
    return node;
}

void preorderTraversal(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    struct TreeNode* node;
    struct StackNode* stack = NULL;
    push(&stack, root);
    while (stack != NULL) {
        node = pop(&stack);
        printf("%d ", node->val);
        if (node->right != NULL) {
            push(&stack, node->right);
        }
        if (node->left != NULL) {
            push(&stack, node->left);
        }
    }
}

void inorderTraversal(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    struct TreeNode* node = root;
    struct StackNode* stack = NULL;
    while (node != NULL || stack != NULL) {
        while (node != NULL) {
            push(&stack, node);
            node = node->left;
        }
        node = pop(&stack);
        printf("%d ", node->val);
        node = node->right;
    }
}

void postorderTraversal(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    struct TreeNode* node = root;
    struct TreeNode* last_visited = NULL;
    struct StackNode* stack = NULL;
    while (node != NULL || stack != NULL) {
        while (node != NULL) {
            push(&stack, node);
            node = node->left;
        }
        node = stack->node;
        if (node->right == NULL || node->right == last_visited) {
            printf("%d ", node->val);
            pop(&stack);
            last_visited = node;
            node = NULL;
        } else {
            node = node->right;
        }
    }
}

int main() {
    struct TreeNode* root = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->val = 1;

    struct TreeNode* node1 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    node1->val = 2;

    struct TreeNode* node2 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    node2->val = 3;

    struct TreeNode* node3 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    node3->val = 4;

    struct TreeNode* node4 = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    node4->val = 5;

    root->left = node1;
    root->right = node2;
    node1->left = node3;
    node1->right = node4;
    node2->left = NULL;
    node2->right = NULL;
    node3->left = NULL;
    node3->right = NULL;
    node4->left = NULL;
    node4->right = NULL;

    printf("Preorder Traversal: ");
    preorderTraversal(root);
    printf("\n");

    printf("Inorder Traversal: ");
    inorderTraversal(root);
    printf("\n");

    printf("Postorder Traversal: ");
    postorderTraversal(root);
    printf("\n");

    return 0;
}

对于中序遍历和后序遍历，只需要修改遍历顺序即可。