Merge pull request #922 from qxuewei/master

添加 150. 逆波兰表达式求值 Swift版本

problems/0150.逆波兰表达式求值.md

@@ -241,6 +241,38 @@ class Solution:
 
 ```
 
+Swift：
+```Swift
+func evalRPN(_ tokens: [String]) -> Int {
+    var stack = [Int]()
+    for c in tokens {
+        let v = Int(c)
+        if let num = v {
+            // 遇到数字直接入栈
+            stack.append(num)
+        } else {
+            // 遇到运算符, 取出栈顶两元素计算, 结果压栈
+            var res: Int = 0
+            let num2 = stack.popLast()!
+            let num1 = stack.popLast()!
+            switch c {
+            case "+":
+                res = num1 + num2
+            case "-":
+                res = num1 - num2
+            case "*":
+                res = num1 * num2
+            case "/":
+                res = num1 / num2
+            default:
+                break
+            }
+            stack.append(res)
+        }
+    }
+    return stack.last!
+}
+```
 
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