feat: add 020

Author: Blankj

README.md

@@ -13,6 +13,7 @@
 |13|[Roman to Integer][013]|Math, String|
 |14|[Longest Common Prefix][014]|String|
 |19|[Remove Nth Node From End of List][019]|Linked List, Two Pointers|
+|20|[Valid Parentheses][020]|Stack, String|
 
 
 ## Medium
@@ -40,3 +41,4 @@
 [013]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/013/README.md
 [014]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/014/README.md
 [019]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/019/README.md
+[020]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/020/README.md

note/020/README.md

@@ -0,0 +1,47 @@
+# [Valid Parentheses][title]
+
+## Description
+
+Given a string containing just the characters `'('`, `')'`, `'{'`, `'}'`, `'['` and `']'`, determine if the input string is valid.
+
+The brackets must close in the correct order, `"()"` and `"()[]{}"` are all valid but `"(]"` and `"([)]"` are not.
+
+**Tags:** Stack, String
+
+## 思路
+
+题意是判断括号匹配是否正确，很明显，我们可以用栈来解决这个问题，当出现左括号的时候入栈，当遇到右括号时，判断栈顶的左括号是否何其匹配，不匹配的话直接返回`false`即可，最终判断是否空栈即可，这里我们可以用数组模拟栈的操作使其操作更快，有个细节注意下`top =  1;`，从而省去了之后判空的操作和`top - 1`导致数组越界的错误。
+
+``` java
+public class Solution {
+    public boolean isValid(String s) {
+        int len = s.length();
+        char[] stack = new char[len + 1];
+        int top = 1;
+        for (int i = 0; i < len; ++i) {
+            char c = s.charAt(i);
+            if (c == '(' || c == '[' || c == '{')
+                stack[top++] = c;
+            else if (c == ')' && stack[top - 1] != '(')
+                return false;
+            else if (c == ']' && stack[top - 1] != '[')
+                return false;
+            else if (c == '}' && stack[top - 1] != '{')
+                return false;
+            else
+                --top;
+        }
+        return top == 1;
+    }
+}
+```
+
+
+## 结语
+
+如果你同我一样热爱数据结构、算法、LeetCode，可以关注我GitHub上的LeetCode题解：[awesome-java-leetcode][ajl]
+
+
+
+[title]: https://leetcode.com/problems/valid-parentheses
+[ajl]: https://github.com/Blankj/awesome-java-leetcode

project/LeetCode/leetcode/src/main/java/com/blankj/easy/_020/Solution.java

@@ -0,0 +1,39 @@
+package com.blankj.easy._020;
+
+import java.util.Arrays;
+import java.util.HashMap;
+
+/**
+ * <pre>
+ *     author: Blankj
+ *     blog  : http://blankj.com
+ *     time  : 2017/04/21
+ *     desc  :
+ * </pre>
+ */
+
+public class Solution {
+    public boolean isValid(String s) {
+        int len = s.length();
+        char[] stack = new char[len + 1];
+        int top = 1;
+        for (int i = 0; i < len; ++i) {
+            char c = s.charAt(i);
+            if (c == '(' || c == '[' || c == '{')
+                stack[top++] = c;
+            else if (c == ')' && stack[top - 1] != '(')
+                return false;
+            else if (c == ']' && stack[top - 1] != '[')
+                return false;
+            else if (c == '}' && stack[top - 1] != '{')
+                return false;
+            else
+                --top;
+        }
+        return top == 1;
+    }
+
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+    }
+}