20. Valid Parentheses

20/1.cpp

@@ -0,0 +1,20 @@
+class Solution {
+public:
+    bool isValid(string s) {
+        stack<char> stack;
+
+        for(char c : s){
+            // stack topとのチェック
+            if(!stack.empty() && isPair(stack.top(), c)){
+                stack.pop();
+            }else{
+                stack.push(c);
+            }
+        }
+        return stack.empty();
+    }
+
+    bool isPair(char l, char r){
+        return (l == '(' && r == ')' || l == '{' && r == '}' || l == '[' && r == ']');
+    }
+};

20/2.cpp

@@ -0,0 +1,19 @@
+class Solution {
+public:
+    bool isValid(string s) {
+        stack<char> stack;
+
+        for(char c : s){
+            if(!stack.empty() && isPair(stack.top(), c)){
+                stack.pop();
+            }else{
+                stack.push(c);
+            }
+        }
+        return stack.empty();
+    }
+
+    bool isPair(char l, char r){
+        return (l == '(' && r == ')' || l == '{' && r == '}' || l == '[' && r == ']');
+    }
+};

20/3.cpp

@@ -0,0 +1,20 @@
+class Solution {
+public:
+    bool isValid(string s) {
+        stack<char> stack;
+
+        for(char c: s){
+            if(!stack.empty() && isPair(stack.top(), c)){
+                stack.pop();
+            }else{
+                stack.push(c);
+            }
+        }
+
+        return stack.empty();
+    }
+
+    bool isPair(char l, char r){
+        return (l == '{' && r == '}' || l == '(' && r == ')' || l == '[' && r == ']');
+    }
+};

20/4.cpp

@@ -0,0 +1,29 @@
+class Solution {
+public:
+    bool isValid(string s) {
+        stack<char> stack;
+
+        if(s[0] == ')' || s[0] == '}' || s[0] == ']'){
+            return false;
+        }
+
+        for(char c : s){
+            if(!stack.empty() && isInvalidPair(stack.top(), c)){return false;}
+
+            if(c != '(' && c != '{' && c != '[' && !stack.empty() && isPair(stack.top(), c)){
+                stack.pop();
+            }else{
+                stack.push(c);
+            }
+        }
+        return stack.empty();
+    }
+
+    bool isPair(char l, char r){
+        return (l == '(' && r == ')' || l == '{' && r == '}' || l == '[' && r == ']');
+    }
+
+    bool isInvalidPair(char l, char r){
+        return (l == '(' && (r == '}' || r == ']')) || (l == '{' && (r == ')' || r == ']')) || (l == '[' && (r == '}' || r == ')'));
+    }
+};