LongestValidParentheses

Author: zgpeace

code/LeetCode/src/popular/LongestValidParentheses.java

@@ -0,0 +1,82 @@
+package popular;
+
+import java.util.Stack;
+
+public class LongestValidParentheses {
+
+    /**
+     * 用Stack存储开始匹配的上一个位置，那么当前的位置减去上个位置，就是最长的匹配,
+     * 时间复杂度为O(n), 空间复杂度为O(n)
+     * @param s
+     * @return
+     */
+    public int longestValidParentheses(String s) {
+        int len = s.length();
+        Stack<Integer> stack = new Stack<>();
+        stack.push(-1);
+        int maxLen = 0;
+        for (int i = 0; i < len; i++) {
+            if (s.charAt(i) == '(') {
+                stack.push(i);
+            } else {
+                stack.pop();
+                if (stack.isEmpty()) {
+                    stack.push(i);
+                } else {
+                    maxLen = Math.max(maxLen, i - stack.peek());
+                }
+            }
+        }
+
+        return maxLen;
+    }
+
+
+    /**
+     * 从左到右，又从右到左，两个方向找最长的匹配。
+     * 时间复杂度为O(n), 空间复杂度为O(1)
+     * @param s
+     * @return
+     */
+    public int longestValidParenthesesWithoutStack(String s) {
+        int left = 0;
+        int right = 0;
+        int maxLen = 0;
+        int len = s.length();
+        for (char c: s.toCharArray()) {
+            if (c == '(') {
+                left++;
+            } else {
+                right++;
+            }
+            if (left == right) {
+                maxLen = Math.max(maxLen, right * 2);
+            } else if (right > left) {
+                left = right = 0;
+            }
+        }
+
+        left = right = 0;
+        for (int i = len - 1; i >= 0; i--) {
+            if (s.charAt(i) == '(') {
+                left++;
+            } else {
+                right++;
+            }
+            if (left == right) {
+                maxLen = Math.max(maxLen, left * 2);
+            } else if (left > right) {
+                left = right = 0;
+            }
+        }
+
+        return maxLen;
+    }
+
+    public static void main(String[] args) {
+        String input = "((())";
+        LongestValidParentheses obj = new LongestValidParentheses();
+        //System.out.println("input: ((()), Output -> " + obj.longestValidParentheses(input));
+        System.out.println("input: ((()), Output -> " + obj.longestValidParenthesesWithoutStack(input));
+    }
+}