From a66e2251c0e053438c6e15057021df7d1b44e375 Mon Sep 17 00:00:00 2001
From: Jidong Xiao <xiaoj8@rpi.edu>
Date: Sat, 11 Nov 2023 21:53:04 -0500
Subject: [PATCH] adding two stack problems

---
 leetcode/p20_valid_parentheses.cpp | 37 ++++++++++++++++++++++++++++++
 leetcode/p71_simplify_path.cpp     | 30 ++++++++++++++++++++++++
 2 files changed, 67 insertions(+)
 create mode 100644 leetcode/p20_valid_parentheses.cpp
 create mode 100644 leetcode/p71_simplify_path.cpp

diff --git a/leetcode/p20_valid_parentheses.cpp b/leetcode/p20_valid_parentheses.cpp
new file mode 100644
index 0000000..3e7f055
--- /dev/null
+++ b/leetcode/p20_valid_parentheses.cpp
@@ -0,0 +1,37 @@
+class Solution {
+public:
+    bool isValid(string s) {
+        std::stack<char> myStack;
+        int len = s.length();
+        char c;
+        for(int i=0;i<len;i++){
+            // push all the open brackets into the stack
+            if(s[i]=='(' || s[i]=='{' || s[i]=='['){
+                myStack.push(s[i]);
+            }else{
+                // if we encounter a close bracket first, it's already invalid.
+                if(myStack.empty()){
+                    return false;
+                }
+                c = myStack.top();
+                myStack.pop();
+                // for every close bracket we encounter, there must be a corresponding open bracket at the top of the stack.
+                if(s[i]==')' && c!='('){
+                    return false;
+                }
+                if(s[i]=='}' && c!='{'){
+                    return false;
+                }
+                if(s[i]==']' && c!='['){
+                    return false;
+                }
+            }
+        }
+        // if it's empty, we are good.
+        if(myStack.empty()){
+            return true;
+        }else{
+            return false;
+        }
+    }
+};
diff --git a/leetcode/p71_simplify_path.cpp b/leetcode/p71_simplify_path.cpp
new file mode 100644
index 0000000..2c6c8c5
--- /dev/null
+++ b/leetcode/p71_simplify_path.cpp
@@ -0,0 +1,30 @@
+class Solution {
+public:
+    string simplifyPath(string path) {
+        std::string item;
+        std::stack<string> myStack;
+        // convert path to a string stream, and split it by the delimiter '/'.
+        stringstream ss(path);
+        while(getline(ss, item, '/')){
+            if(item == ".."){
+                if(!myStack.empty()){
+                    myStack.pop();
+                }
+            }else if(item != "." && item != ""){
+                // we only push valid items into the stack
+                myStack.push(item);
+            }
+        }
+        // since we were given an absolute path, which starts with a slash, if at this moment the stack is empty, it means either we didn't push anything into the stack, or everything is popped out.
+        if(myStack.empty()){
+            return "/";
+        }
+        std::string result = "";
+        // now let's concatenate all valid items.
+        while(!myStack.empty()){
+            result = "/" + myStack.top() + result;
+            myStack.pop();
+        }
+        return result;
+    }
+};