中缀表达式 转换成 前缀表达式（二叉树实现）

#include <stdio.h>#include <stdlib.h>#include <string.h>#define N 100typedef struct Node{ char key; struct Node * left;struct Node * right;}Node;//search for the operator with the highest grade in aint search(char a[], int begin, int end){ int tag = -1;int isInBrackets = 0;if(a[begin] == '(' && a[end] == ')'){ begin += 1;end -= 1;}int amExist = 0;for(int i = begin; i < end; i++){ if(a[i] == '(') isInBrackets++; if(a[i] == ')') isInBrackets--;if((a[i] == '+' || a[i] == '-') && isInBrackets == 0){ tag = i;amExist = 1;}if((a[i] == '*' || a[i] == '/') && isInBrackets == 0 && amExist == 0){ tag = i;}}return tag;}//build the binary treeNode * buildBinaryTree(char a[], int begin, int end){Node * p = NULL; if(begin == end){ p = (Node *)malloc(sizeof(Node)); p->key = a[begin]; p->left = NULL; p->right = NULL;}else{ int tag;tag = search(a,begin,end); p = (Node *)malloc(sizeof(Node));p->key = a[tag];if(a[begin] == '(' && a[end] == ')'){ begin += 1;end -= 1;}p->left = buildBinaryTree(a, begin, tag - 1);p->right = buildBinaryTree(a, tag + 1, end );} return p;}void outputBinaryTree(Node * head){ if(head){ printf("%c", head->key);outputBinaryTree(head->left);outputBinaryTree(head->right);}}void freeBinaryTree(Node * head){ if(head){ Node *left = head->left;Node *right = head->right;free(head);head = NULL;freeBinaryTree(left);freeBinaryTree(right);}}int main(){printf("please input a expression:");char input[N]; while(scanf("%s", input)){ if(strcmp(input, "exit") == 0){ break;}Node * head = NULL;int length = strlen(input);head = buildBinaryTree(input, 0, length - 1);outputBinaryTree(head);printf("/n");freeBinaryTree(head);printf("please input a expression:"); }return 0;}