[LeetCode] Valid Parentheses

AlphaWallet 發(fā)布于2019-08-14 14:18 / 2935人閱讀

摘要：循環(huán)每個(gè)元素放入堆棧或比較是否和棧頂元素成對(duì)。循環(huán)結(jié)束后，若未拋出，且堆棧為空，說(shuō)明所有都已一一對(duì)應(yīng)。

Problem

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.

Note

建立堆棧stack。循環(huán)每個(gè)元素放入堆棧或比較是否和棧頂元素成對(duì)。
循環(huán)結(jié)束后，若未拋出false，且堆棧為空，說(shuō)明所有parenthese都已一一對(duì)應(yīng)。

Solution HashMap+Switch/case

public class Solution {
    public boolean isValid(String s) {
        Map map = new HashMap<>();
        map.put("(", ")");
        map.put("[", "]");
        map.put("{", "}");
        Stack stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            Character ch = s.charAt(i);
            switch (ch) {
                case "{": case "[": case "(":
                    stack.push(ch);
                    break;
                case ")": case "}": case "]":
                    if (stack.isEmpty() || ch != map.get(stack.pop())) return false;
            }
        }
        return stack.isEmpty();
    }
}

if-else branches

public class Solution {
    /**
     * @param s: A string
     * @return: whether the string is a valid parentheses
     */
    public boolean isValidParentheses(String s) {
        char[] str = s.toCharArray();
        if (str.length % 2 != 0) return false;
        Stack stack = new Stack<>();
        for (char ch: str) {
            if (ch == "(" || ch == "[" || ch == "{") {
                stack.push(ch);
            } else {
                if (stack.isEmpty()) {
                    return false;
                } else {
                    char top = stack.pop();
                    if ((ch == ")" && top != "(") || (ch == "]" && top != "[") || (ch == "}" && top != "{")) {
                        return false;
                    }
                }
            }
        }
        return stack.isEmpty();
    }
}