add RPN snippets

snippets/postfixToInfix.md

@@ -0,0 +1,29 @@
+const postFix = RPN => {
+  let convert = RPN.replace(/\^/g,'**').split(/\s+/g).filter(el => !/\s+/.test(el) && el !== '')
+  let stack = []
+  let result = []
+  let friends = {"+" : ["+","-","*","/"],"-":[],"/":["*"],"*":["/","*"],"**":["+","-","*","/"]}
+  convert.forEach(symbol => {
+    if(!isNaN(parseFloat(symbol)) && isFinite(symbol)){
+      result.push(symbol)
+    }
+    else if (Object.keys(friends).includes(symbol)) {
+      a = result.pop()
+      b = result.pop()
+      if(stack.length !==0){
+          if(friends[symbol].includes(stack.pop())){
+            result.push(`${b} ${symbol} ${a}`)
+            stack.push(symbol)
+          }
+          else{
+            result.push(`(${b}) ${symbol} ${a}`)
+            stack.push(symbol)
+          }
+      }
+      else {result.push(`${b} ${symbol} ${a}`);stack.push(symbol)}
+    }
+    else throw `${symbol} is not a recognized symbol`
+  })
+  if(result.length === 1) return result.pop()
+  else throw `${RPN} is not a correct RPN`
+}