From 0f40c563d79ab019c9b99e460d2a5b264b41ce5c Mon Sep 17 00:00:00 2001
From: SinnoLn <sintmtn69@gmail.com>
Date: Sat, 10 Oct 2026 17:56:47 +0900
Subject: [PATCH 1/2] 20. Valid Parentheses

---
 .../20. Valid Parentheses.java"               | 40 +++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100644 "leetcode3/\354\235\264\354\247\204\355\235\254/20. Valid Parentheses.java"

diff --git "a/leetcode3/\354\235\264\354\247\204\355\235\254/20. Valid Parentheses.java" "b/leetcode3/\354\235\264\354\247\204\355\235\254/20. Valid Parentheses.java"
new file mode 100644
index 00000000..1f1f9999
--- /dev/null
+++ "b/leetcode3/\354\235\264\354\247\204\355\235\254/20. Valid Parentheses.java"	
@@ -0,0 +1,40 @@
+/*
+
+1. 아이디어 : stack 구조 성질 활용
+             push, pop을 하며 괄호가 올바르게 배치됐는지 체크
+
+2. 시간복잡도 : O(N)
+
+3. 자료구조/알고리즘 : stack
+
+ */
+
+class Solution {
+    public boolean isValid(String s) {
+        // stack 구조 활용
+
+        Deque<Character> stack = new ArrayDeque<>();
+
+        for(int i=0; i<s.length(); i++) {
+            char c = s.charAt(i);
+
+            if(c=='(' || c=='[' || c=='{') stack.add(c);
+            else {
+                if(stack.isEmpty()) return false;
+                
+                char prev = stack.pollLast();
+
+                if(prev == '(' && c == ')') continue;
+                if(prev == '{' && c == '}') continue;
+                if(prev == '[' && c == ']') continue;
+
+                return false;
+            }
+
+        }
+
+        if(!stack.isEmpty()) return false;
+
+        return true;
+    }
+}
\ No newline at end of file

From 39c7305cdfaaea00583b7b56db9c344a904b7944 Mon Sep 17 00:00:00 2001
From: SinnoLn <sintmtn69@gmail.com>
Date: Sat, 10 Oct 2026 17:58:52 +0900
Subject: [PATCH 2/2] 593. Valid Square

---
 .../593. Valid Square.java"                   | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)
 create mode 100644 "leetcode3/\354\235\264\354\247\204\355\235\254/593. Valid Square.java"

diff --git "a/leetcode3/\354\235\264\354\247\204\355\235\254/593. Valid Square.java" "b/leetcode3/\354\235\264\354\247\204\355\235\254/593. Valid Square.java"
new file mode 100644
index 00000000..3571df2f
--- /dev/null
+++ "b/leetcode3/\354\235\264\354\247\204\355\235\254/593. Valid Square.java"	
@@ -0,0 +1,38 @@
+/*
+1. 아이디어: 4점이 주어졌을때 정사각형 판별 - 정사각형의 성질 활용
+           1. 4변의 길이가 같다 
+           2. 두 대각선의 길이가 같다
+
+2. 시간복잡도: O(1);
+
+3. 자료구조/알고리즘: 정렬, 정사각형 성질(수학)
+
+*/
+
+class Solution {
+    public boolean validSquare(int[] p1, int[] p2, int[] p3, int[] p4) {
+
+        int[] dis = new int[6];
+        dis[0] = dist(p1, p2);
+        dis[1] = dist(p1, p3);
+        dis[2] = dist(p1, p4);
+        dis[3] = dist(p2, p3);
+        dis[4] = dist(p2, p4);
+        dis[5] = dist(p3, p4);
+        
+
+        // 마지막 2개 원소가 대각선
+        Arrays.sort(dis);
+
+        if(dis[0] == 0) return false;
+        
+        return dis[0] == dis[1] &&
+               dis[1] == dis[2] &&
+               dis[2] == dis[3] &&
+               dis[4] == dis[5];
+    }
+    
+    private int dist(int[] a, int[] b) {
+        return (a[0] - b[0]) * (a[0] - b[0]) + (a[1] - b[1]) * (a[1] - b[1]);
+    }
+}
\ No newline at end of file