subtree(users/wpcarro): docking briefcase at '24f5a642' r/3226

git-subtree-dir: users/wpcarro
git-subtree-mainline: 464bbcb15c09813172c79820bcf526bb10cf4208
git-subtree-split: 24f5a642af3aa1627bbff977f0a101907a02c69f
Change-Id: I6105b3762b79126b3488359c95978cadb3efa789

1 files changed, 51 insertions, 0 deletions

diff --git a/users/wpcarro/scratch/facebook/infix-to-postfix.py b/users/wpcarro/scratch/facebook/infix-to-postfix.py
new file mode 100644
index 000000000000..4c6d64494d95
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/infix-to-postfix.py
@@ -0,0 +1,51 @@
+operators = {
+    '*': 1,
+    '+': 0,
+}
+
+def tokenize(xs):
+    result = []
+    i = 0
+    while i < len(xs):
+        current = xs[i]
+        if current in operators.keys():
+            result.append(current)
+            i += 1
+            continue
+        else:
+            i += 1
+            while i < len(xs) and xs[i] in {str(n) for n in range(10)}:
+                current += xs[i]
+                i += 1
+            result.append(int(current))
+    return result
+
+def postfix(xs):
+    result = []
+    s = []
+    for x in xs:
+        if x in operators.keys():
+            while s and operators[s[-1]] >= operators[x]:
+                result.append(s.pop())
+            s.append(x)
+        else:
+            result.append(x)
+    while s:
+        result.append(s.pop())
+    return result
+
+def evaluate(xs):
+    s = []
+    for x in xs:
+        print(s, x)
+        if x == '*':
+            s.append(s.pop() * s.pop())
+        elif x == '+':
+            s.append(s.pop() + s.pop())
+        else:
+            s.append(x)
+        print(s)
+    return s[-1]
+
+
+print(evaluate(postfix(tokenize("12+3*10"))))