Evaluate reverse polish notation

Signed-off-by: Leo Ma <begeekmyfriend@gmail.com>

Author: begeekmyfriend

150_evaluate_reverse_polish_notation/Makefile

@@ -0,0 +1,2 @@
+all:
+	gcc -O2 -o test eval_rpn.c

150_evaluate_reverse_polish_notation/eval_rpn.c

@@ -0,0 +1,87 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+static int a2i(char *s)
+{
+    int n = 0;
+    int sign = 0;
+    while (*s != '\0') {
+        switch (s[0]) {
+        case '+':
+            break;
+        case '-':
+            sign = 1;
+            break;
+        case '*':
+            break;
+        case '/':
+            break;
+        default:
+            n = n * 10 + (s[0] - '0');
+            break;
+        }
+        s++;
+    }
+    return sign ? -n : n;
+}
+
+static int evalRPN(char **tokens, int tokensSize)
+{
+    int i, res, a, b;
+    int stack[500], len = 0;
+
+    for (i = 0; i < tokensSize; i++) {
+        switch (tokens[i][0]) {
+        case '\0':
+            break;
+        case '+':
+            if (len > 0) {
+                b = stack[--len];
+            }
+            if (len > 0) {
+                a = stack[--len];
+                stack[len++] = a + b;
+            }
+            break;
+        case '*':
+            if (len > 0) {
+                b = stack[--len];
+            }
+            if (len > 0) {
+                a = stack[--len];
+                stack[len++] = a * b;
+            }
+            break;
+        case '/':
+            if (len > 0) {
+                b = stack[--len];
+            }
+            if (len > 0) {
+                a = stack[--len];
+                stack[len++] = a / b;
+            }
+            break;
+        case '-':
+            if (tokens[i][1] == '\0') {
+                if (len > 0) {
+                    b = stack[--len];
+                }
+                if (len > 0) {
+                    a = stack[--len];
+                    stack[len++] = a - b;
+                }
+                break;
+            }
+        default:
+            stack[len++] = a2i(tokens[i]);
+        }
+    }
+
+    return len > 0 ? stack[len - 1] : 0;
+}
+
+int main(int argc, char **argv)
+{
+    printf("%d\n", evalRPN(argv + 1, argc - 1));
+    return 0;
+}