Create build-binary-expression-tree-from-infix-expression.cpp

Author: kamyu104

C++/build-binary-expression-tree-from-infix-expression.cpp

@@ -0,0 +1,49 @@
+// Time:  O(n)
+// Space: O(n)
+
+// Support +, -, *, /.
+class Solution {
+public:
+    Node* expTree(string s) {
+        static const unordered_map<char, int> precedence = {{'+', 0}, {'-', 0}, {'*', 1}, {'/', 1}};
+
+        stack<Node *> operands;
+        stack<char> operators;
+        int64_t operand = 0;
+        for (int i = 0; i < size(s); ++i) {
+            if (isdigit(s[i])) {
+                operand = operand * 10 + s[i] - '0';
+                if (i + 1 == size(s) || !isdigit(s[i + 1])) {
+                    operands.emplace(new Node(operand + '0'));
+                    operand = 0;
+                }
+            } else if (s[i] == '(') {
+                operators.emplace(s[i]);
+            } else if (s[i] == ')') {
+                while (!empty(operators) && operators.top() != '(') {
+                    compute(&operands, &operators);
+                }
+                operators.pop();
+            } else if (precedence.count(s[i])) {
+                while (!empty(operators) && precedence.count(operators.top()) && 
+                       precedence.at(operators.top()) >= precedence.at(s[i])) {
+                     compute(&operands, &operators);
+                }
+                operators.emplace(s[i]);
+            }
+        }
+        while (!empty(operators)) {
+            compute(&operands, &operators);
+        }
+        return operands.top();
+    }
+
+private:
+    template<typename T>
+    void compute(stack<T> *operands, stack<char> *operators) {
+        const auto right = operands->top(); operands->pop();
+        const auto left = operands->top(); operands->pop();
+        const char op = operators->top(); operators->pop();
+        operands->emplace(new Node(op, left, right));
+    }
+};