150 solved. (#48)

Author: zwfang

README.md

@@ -110,6 +110,7 @@ continually updating 😃.
 * [9. Palindrome Number](src/0009_palindrome_number/palindrome_number.go)
 * [13. Roman to Integer](src/0013_roman_to_integer/roman_to_integer.go)   *`string`*
 * [66. Plus One](src/0066_plus_one/plus_one.go)   *`array`*
+* [150. Evaluate Reverse Polish Notation](src/0150_evaluate_reverse_polish_notation/evaluate_reverse_polish_notation.go)   *`stack`*
 
 <details>
 </details>

src/0150_evaluate_reverse_polish_notation/evaluate_reverse_polish_notation.go

@@ -0,0 +1,47 @@
+/*
+150. Evaluate Reverse Polish Notation
+https://leetcode.com/problems/evaluate-reverse-polish-notation/
+
+Evaluate the value of an arithmetic expression in Reverse Polish Notation.
+
+Valid operators are +, -, *, /. Each operand may be an integer or another expression.
+
+Note:
+
+Division between two integers should truncate toward zero.
+The given RPN expression is always valid.
+That means the expression would always evaluate to a result and there won't be any divide by zero operation.
+*/
+// time: 2019-01-07
+
+package evaluatereversepolishnotation
+
+import "strconv"
+
+// stack
+// time complexity: O(n)
+// space complexity: O(n)
+func evalRPN(tokens []string) int {
+	stack := make([]int, len(tokens))
+	top := -1
+	for i := 0; i < len(tokens); i++ {
+		switch ch := tokens[i]; ch {
+		case "+":
+			stack[top-1] += stack[top]
+			top--
+		case "-":
+			stack[top-1] -= stack[top]
+			top--
+		case "*":
+			stack[top-1] *= stack[top]
+			top--
+		case "/":
+			stack[top-1] /= stack[top]
+			top--
+		default:
+			top++
+			stack[top], _ = strconv.Atoi(ch)
+		}
+	}
+	return stack[0]
+}

src/0150_evaluate_reverse_polish_notation/evaluate_reverse_polish_notation_test.go

@@ -0,0 +1,20 @@
+package evaluatereversepolishnotation
+
+import "testing"
+
+func TestEvalRPN(t *testing.T) {
+	testCases := [][]string{
+		{"2", "1", "+", "3", "*"},
+		{"4", "13", "5", "/", "+"},
+		{"10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"},
+		{"4", "3", "-"},
+	}
+
+	expected := []int{9, 6, 22, 1}
+
+	for index, tokens := range testCases {
+		if res := evalRPN(tokens); res != expected[index] {
+			t.Errorf("expected %d, got %d", expected[index], res)
+		}
+	}
+}

src/README.md

@@ -58,6 +58,7 @@
 |0122|[122. Best Time to Buy and Sell Stock II](0122_best_time_to_buy_and_sell_stock_2/maxprofit.go)|Easy|*`greedy`*|
 |0125|[Valid Palindrome](0125_valid_palindrome/valid_palindrome.go)|Easy||
 |0144|[144. Binary Tree Preorder Traversal](0144_binary_tree_preorder_traversal/binary_tree_preorder_traversal.go)|Medium|*`binary tree`*|
+|0150|[150. Evaluate Reverse Polish Notation](0150_evaluate_reverse_polish_notation/evaluate_reverse_polish_notation.go)|Medium|*`stack`*|
 |0167|[Two Sum II - Input array is sorted](./0167_two_sum2/two_sum2.go)|Easy|*`对撞指针(双索引)`*|
 |0198|[House Robber](./0198_house_robber/house_robber.go)|Easy|*`memory search;`* *`dynamic programming`*|
 |0209|[Minimum Size Subarray Sum](./0209_minimum_size_subarray_sum/minimum_size_subarray_sum.go)|Medium|*`sliding window`*|