建算数表达式树并计算(应付作业，仅供参考)

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只是为了应付数据结构老师布置的作业，算式里的数只能是整数：

例如输入 (6+3)*(4-2)*(41-1)

输出 720.00

#include <stdio.h>#include <string.h>const int N = 1000;int m[N], flag;double res;struct Node {double val;char c;Node* left;Node* right;Node() {left = right = NULL;c = 'a' - 1;}};Node* build(char *str, int l, int r) {int flag1 = -1, flag2 = -1, p = 0;Node* node = new Node;if (r - l == 1) {node -> val = m[str[l]];return node;}for (int i = l; i < r; i++) {switch(str[i]) {case '(' : p++; break;case ')' : p--; break;case '+' :case '-' :if (!p)flag1 = i;break;case '*' :case '/' :if (!p)flag2 = i;break;}}if (flag1 < 0)flag1 = flag2;if (flag1 < 0)return build(str, l + 1, r - 1);node -> left = build(str, l, flag1);node -> right = build(str, flag1 + 1, r);node -> c = str[flag1];return node;}double preOrder(Node* root) {if (root == NULL)return 0;if (root -> c == 'a' - 1)return root -> val;double a;switch(root -> c) {case '+': a = preOrder(root -> left) + preOrder(root -> right);break;case '-': a = preOrder(root -> left) - preOrder(root -> right);break;case '*': a = preOrder(root -> left) * preOrder(root -> right);break;case '/': if (preOrder(root -> right) != 0)a = preOrder(root -> left) / preOrder(root -> right);elseflag = 1;break;}return a;}int main() {Node* root;char str[N], s[N];while (scanf("%s", str) != EOF) {memset(m, 0, sizeof(m));res = flag = 0;int len = strlen(str);int n = 0;char a = 'a';if (str[0] == '-')s[n++] = a++;for (int i = 0; i < len; i++) {if (str[i] > '9' || str[i] < '0')s[n++] = str[i];else {int sum = 0;while (str[i] >= '0' && str[i] <= '9') {sum = sum * 10 + str[i] - '0';i++;}i--;m[a] = sum;s[n++] = a++;}}s[n] = '\0';root = build(s, 0, n);res = preOrder(root);if (!flag)printf("表达式结果： %.2lf\n", res);elseprintf("error!\n");}return 0;}

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