logseq_notes/pages/OJ notes/pages/Leetcode Valid-Parentheses.md

Leetcode Valid-Parentheses

2022-06-16 14:50

Data structures:

#DS #stack

Difficulty:

#coding_problems #difficulty_easy

Additional tags:

#leetcode

Revisions:

1st revision: 2022-07-03 optimize code

Related topics:

Links:

• Link to problem

  ---

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.

Examples

Example 1:

**Input:** s = "()"
**Output:** true

Example 2:

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

Example 3:

**Input:** s = "(]"
**Output:** false

Constraints

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

Thoughts

Basic stack usage. Can be implemented using std_stack or stc_vector. obviously i should use std_stack.

Solution

1st revision: optimized code

class Solution {
public:
bool isValid(string s) {
  stack<char> st;

  char pairs[3][2] = {'(', ')', '{', '}', '[', ']'};
  const int pairSize = 3;

  for (char c : s) {
    for (int i = 0; i < pairSize; i++) {
      if (pairs[i][0] == c) {
        // {([
        st.push(c);
      } else if (pairs[i][1] == c) {
        // ]})
        if (st.empty()) {
          // empty
          return false;
        } else if (st.top() != pairs[i][0]) {
          // mismatch
          return false;
        } else {
          st.pop();
        }
      }
    }
  }

  return st.empty();
}
};

class Solution {
public:
bool isValid(string s) {
  vector<char> stack;
  vector<vector<char>> pairs = {{'(', ')'}, {'{', '}'}, {'[', ']'}};

  int loc;
  const int size = pairs.size();
  for (char c : s) {

    for (loc = 0; loc < size; loc++) {
      if (c == pairs[loc][1]) {
        if (stack.empty()) {
          // if there is no {([ before
          printf("No {([ before.\n");
          return false;
        } else if (stack.back() == pairs[loc][0]) {
          stack.pop_back();
        } else {
          // parens mismatch
          printf("parens mismatch: %c %c\n", stack.back(), pairs[loc][0]);
          return false;
        }
      } else if (c == pairs[loc][0]) {
        stack.push_back(c);
      }
    }
  }

  if (stack.empty()) {
    return true;
  } else {
    return false;
  }
}
};