LeetCode解题报告（273）-- 20. Valid Parentheses

Problem

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Example 1:

Input: s = "()"
Output: true

Example 2:

Input: s = "()[]{}"
Output: true

Example 3:

Input: s = "(]"
Output: false

Example 4:

Input: s = "([)]"
Output: false

Example 5:

Input: s = "{[]}"
Output: true

Constraints:

• 1 <= s.length <= 104
• s consists of parentheses only '()[]{}'.

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Analysis

  这是一道很经典的stack应用题目了，利用栈的特性做括号匹配校验。遇到左边的括号就入栈，遇到右边的括号就和栈顶做匹配，一旦不符合就return false。特别需要注意两种情况：一个是遍历过程中栈已经为空而且出现右边的括号；另一个是遍历完所有后栈还有元素。这两种情况都是无法匹配的，要return false。

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Solution

  无

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Code

class Solution {
public:
    bool isValid(string s) {
        int size = s.size();
        stack<char> st;
        for (int i = 0; i < size; i++) {
            if (s[i] == '(' || s[i] == '[' || s[i] == '{') {
                st.push(s[i]);
            } else {
                if (st.empty()) {
                    return false;
                }
                char ch = st.top();
                st.pop();
                if (ch == '(' && s[i] == ')' || ch == '[' && s[i] == ']' || ch == '{' && s[i] == '}') {
                    continue;
                } else {
                    return false;
                }
            }
        }
        return st.empty();
    }
};

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Summary

  这道题是比较经典的stack题目，这道题目其实就是算术表达式的实现原理，掌握这道题目对理解stack有很好的帮助。这道题这道题目的分享到这里，谢谢您的支持！