150. 逆波兰表达式求值

题意：

解题思路：

思路：（栈）
1. 遍历数组，依次入栈，遇到运算符，就弹出栈顶2个元素进行运算，最后将结果入栈；
注意：为了方便晕眩，第一个出栈元素放运算符右边
["4","13","5","/","+"]=> [4,13,5,/] => [4,2] => [4,2,+] => [6];

PHP代码实现：

class Solution {
    /**
     * @param String[] $tokens
     * @return Integer
     */
    function evalRPN($tokens) {
        $stack = [];
        foreach ($tokens as $token) {
            switch ($token) {
                case '+':
                    array_push($stack, array_pop($stack) + array_pop($stack));
                    break;
                case '-':
                    array_push($stack, -array_pop($stack) + array_pop($stack));
                    break;
                case '*':
                    array_push($stack, array_pop($stack) * array_pop($stack));
                    break;
                case '/':
                    $tmp = array_pop($stack);
                    array_push($stack, intval(array_pop($stack) / $tmp));
                    break;
                default:
                    array_push($stack, (int)$token);
            }
        }
        return array_pop($stack);
    }
}

GO代码实现：

func evalRPN(tokens []string) int {
    stack := []int{}
    for _, val := range tokens {
        l := len(stack)
        switch val {
        case "+":
            stack = append(stack[:l-2], stack[l-2] + stack[l-1])
        case "-":
            stack = append(stack[:l-2], stack[l-2] - stack[l-1])
        case "*":
            stack = append(stack[:l-2], stack[l-2] * stack[l-1])
        case "/":
            stack = append(stack[:l-2], stack[l-2] / stack[l-1])
        default:
            num, _ := strconv.Atoi(val)
            stack = append(stack, num)
        }
    }
    return stack[0]
}