basic-calculator-ii

Author: MaskRay

README.md

@@ -2,7 +2,7 @@
 
 [LeetCode solutions](http://maskray.me/blog/2014-06-29-leetcode-solutions) gives some thoughts on selected problems.
 
-Solved 201/210 problems.
+Solved 202/211 problems.
 
 ## Database
 
@@ -12,6 +12,7 @@ See [database.md](database.md)
 
 | # | Title | Solution |
 |---| ----- | -------- |
+|227|[Basic Calculator II](/problems/basic-calculator-ii/)|[basic-calculator-ii.cc](basic-calculator-ii.cc)|
 |226|[Invert Binary Tree](/problems/invert-binary-tree/)|[invert-binary-tree.cc](invert-binary-tree.cc)|
 |225|[Implement Stack using Queues](/problems/implement-stack-using-queues/)|[implement-stack-using-queues.cc](implement-stack-using-queues.cc)|
 |224|[Basic Calculator](/problems/basic-calculator/)|[basic-calculator.cc](basic-calculator.cc)|
@@ -222,4 +223,3 @@ See [database.md](database.md)
 |3|[Longest Substring Without Repeating Characters](/problems/longest-substring-without-repeating-characters/)|[longest-substring-without-repeating-characters.cc](longest-substring-without-repeating-characters.cc)|
 |2|[Add Two Numbers](/problems/add-two-numbers/)|[add-two-numbers.cc](add-two-numbers.cc)|
 |1|[Two Sum](/problems/two-sum/)|[two-sum.cc](two-sum.cc)|
-

basic-calculator-ii.cc

@@ -0,0 +1,43 @@
+// Basic Calculator II
+// operator-precedence parser
+class Solution
+{
+public:
+  int calculate(string s) {
+    stack<int> xs;
+    stack<char> ops;
+    map<char, int> isp, icp;
+    isp['\0'] = 0; isp['+'] = 3; isp['-'] = 3; isp['*'] = 5; isp['/'] = 5; isp['('] = 1;
+    icp['\0'] = 0; icp['+'] = 2; icp['-'] = 2; icp['*'] = 4; icp['/'] = 4; icp['('] = 6; icp[')'] = 1;
+    ops.push('\0');
+    for (size_t i = 0; i <= s.size(); ) {
+      char op = i == s.size() ? '\0' : s[i];
+      if (op == ' ')
+        i++;
+      else if (isdigit(op)) {
+        int x = 0;
+        do x = x*10+s[i]-'0';
+        while (isdigit(s[++i]));
+        xs.push(x); // shift
+      } else {
+        i++;
+        int x, y;
+        for (; isp[ops.top()] > icp[op]; ops.pop()) {
+          y = xs.top(); xs.pop();
+          x = xs.top(); xs.pop();
+          switch (ops.top()) {
+          case '+': xs.push(x+y); break;
+          case '-': xs.push(x-y); break;
+          case '*': xs.push(x*y); break;
+          case '/': xs.push(x/y); break;
+          }
+        }
+        if (isp[ops.top()] == icp[op]) // reduce, '(' X ')' => X  or  '\0' X '\0' => X
+          ops.pop();
+        else
+          ops.push(op); // shift
+      }
+    }
+    return xs.top();
+  }
+};