表达式二叉树函数(C实现，实现了通过前缀，中缀，后缀表达式构建二叉树)

以下是一个简单的实现，实现了通过后缀表达式构建二叉树：

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

#define MAX_EXPR_LEN 100

typedef struct TreeNode {
    char val;
    struct TreeNode *left;
    struct TreeNode *right;
} TreeNode;

typedef struct StackNode {
    TreeNode *val;
    struct StackNode *next;
} StackNode;

typedef struct Stack {
    StackNode *top;
} Stack;

Stack* createStack() {
    Stack *s = (Stack*) malloc(sizeof(Stack));
    s->top = NULL;
    return s;
}

void push(Stack *s, TreeNode *val) {
    StackNode *n = (StackNode*) malloc(sizeof(StackNode));
    n->val = val;
    n->next = s->top;
    s->top = n;
}

TreeNode* pop(Stack *s) {
    if (s->top == NULL) {
        return NULL;
    }
    TreeNode *val = s->top->val;
    StackNode *temp = s->top;
    s->top = s->top->next;
    free(temp);
    return val;
}

TreeNode* buildExprTree(char *expr) {
    Stack *s = createStack();
    int len = strlen(expr);
    for (int i = 0; i < len; i++) {
        char c = expr[i];
        if (isdigit(c)) {
            TreeNode *n = (TreeNode*) malloc(sizeof(TreeNode));
            n->val = c;
            n->left = NULL;
            n->right = NULL;
            push(s, n);
        } else {
            TreeNode *n = (TreeNode*) malloc(sizeof(TreeNode));
            n->val = c;
            n->right = pop(s);
            n->left = pop(s);
            push(s, n);
        }
    }
    TreeNode *root = pop(s);
    free(s);
    return root;
}

void inOrderTraversal(TreeNode *root) {
    if (root == NULL) {
        return;
    }
    inOrderTraversal(root->left);
    printf("%c ", root->val);
    inOrderTraversal(root->right);
}

int main() {
    char expr[MAX_EXPR_LEN];
    printf("请输入后缀表达式：");
    scanf("%s", expr);
    TreeNode *root = buildExprTree(expr);
    printf("中序遍历结果：");
    inOrderTraversal(root);
    printf("\n");
    return 0;
}

这段代码通过一个栈来辅助构建表达式二叉树，对于每个字符，如果是数字则创建一个只包含该数字的叶子节点，如果是运算符则创建一个新的节点，并将栈顶的两个节点分别作为左右子树连接到该节点上，然后将该节点压入栈中。最后，栈中仅剩的一个节点就是构建好的表达式二叉树的根节点。