Added reverse polish notation solution.

README.md

@@ -259,6 +259,7 @@ programming challenges completed in other languages.
 - [Set Matrix Zeroes](https://leetcode.com/problems/set-matrix-zeroes)
 - [Group Anagrams](https://leetcode.com/problems/group-anagrams)
 - [Min Stack](https://leetcode.com/problems/min-stack)
+- [Reverse Polish Notation](https://leetcode.com/problems/evaluate-reverse-polish-notation)
 
 #### Hard
 

src/leetcode/mod.rs

@@ -121,6 +121,7 @@ mod repeat_limit;
 mod repeated_dna;
 mod replace_digits;
 mod return_to_origin;
+mod reverse_polish_notation;
 mod roman_to_int;
 mod rotate_array;
 mod rotate_box;

src/leetcode/reverse_polish_notation.rs

@@ -0,0 +1,44 @@
+pub fn eval_rpn(tokens: Vec<String>) -> i32 {
+    let mut stack = vec![];
+    for token in tokens {
+        match token.as_str() {
+            "+" => {
+                let b = stack.pop().unwrap();
+                let a = stack.pop().unwrap();
+                stack.push(a + b)
+            }
+            "-" => {
+                let b = stack.pop().unwrap();
+                let a = stack.pop().unwrap();
+                stack.push(a - b)
+            }
+            "*" => {
+                let b = stack.pop().unwrap();
+                let a = stack.pop().unwrap();
+                stack.push(a * b)
+            }
+            "/" => {
+                let b = stack.pop().unwrap();
+                let a = stack.pop().unwrap();
+                stack.push(a / b)
+            }
+            _ => stack.push(token.parse::<i32>().unwrap()),
+        }
+    }
+    stack[0]
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::string_vec;
+
+    use super::*;
+
+    #[test]
+    fn evaluates_reverse_ploish_notation_expression() {
+        assert_eq!(15, eval_rpn(string_vec!["5", "10", "+"]));
+        assert_eq!(10, eval_rpn(string_vec!["5", "10", "+", "5", "-"]));
+        assert_eq!(9, eval_rpn(string_vec!["2", "1", "+", "3", "*"]));
+        assert_eq!(6, eval_rpn(string_vec!["4", "13", "5", "/", "+"]))
+    }
+}