g0001_0100.s0032_longest_valid_parentheses.Solution Maven / Gradle / Ivy

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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0001_0100.s0032_longest_valid_parentheses;

// #Hard #Top_100_Liked_Questions #String #Dynamic_Programming #Stack #Big_O_Time_O(n)_Space_O(1)
// #2023_08_09_Time_1_ms_(100.00%)_Space_41.4_MB_(85.22%)

/**
 * 32 - Longest Valid Parentheses\.
 *
 * Hard
 *
 * Given a string containing just the characters `'('` and `')'`, find the length of the longest valid (well-formed) parentheses substring.
 *
 * **Example 1:**
 *
 * **Input:** s = "(()"
 *
 * **Output:** 2
 *
 * **Explanation:** The longest valid parentheses substring is "()". 
 *
 * **Example 2:**
 *
 * **Input:** s = ")()())"
 *
 * **Output:** 4
 *
 * **Explanation:** The longest valid parentheses substring is "()()". 
 *
 * **Example 3:**
 *
 * **Input:** s = ""
 *
 * **Output:** 0 
 *
 * **Constraints:**
 *
 * *   0 <= s.length <= 3 * 10⁴
 * *   `s[i]` is `'('`, or `')'`.
**/
public class Solution {
    public int longestValidParentheses(String s) {
        int max = 0;
        int left = 0;
        int right = 0;
        int n = s.length();
        char ch;
        for (int i = 0; i < n; i++) {
            ch = s.charAt(i);
            if (ch == '(') {
                left++;
            } else {
                right++;
            }
            if (right > left) {
                left = 0;
                right = 0;
            }
            if (left == right) {
                max = Math.max(max, left + right);
            }
        }
        left = 0;
        right = 0;
        for (int i = n - 1; i >= 0; i--) {
            ch = s.charAt(i);
            if (ch == '(') {
                left++;
            } else {
                right++;
            }
            if (left > right) {
                left = 0;
                right = 0;
            }
            if (left == right) {
                max = Math.max(max, left + right);
            }
        }
        return max;
    }
}