Valid Parentheses

Problem (restated)

Given a string of brackets, determine if it is valid (correct nesting and order).

Intuition

Push openers; on closer, top must match.

Approaches

export function isValid(s: string): boolean {
  const st: string[] = [];
  const pair: Record<string, string> = { ')': '(', ']': '[', '}': '{' };
  for (const ch of s) {
    if (ch === '(' || ch === '[' || ch === '{') st.push(ch);
    else {
      if (!st.length || st.pop() !== pair[ch]) return false;
    }
  }
  return st.length === 0;
}

export function isValid(s: string): boolean {
  const st: string[] = [];
  const pair: Record<string, string> = { ')': '(', ']': '[', '}': '{' };
  for (const ch of s) {
    if (ch === '(' || ch === '[' || ch === '{') st.push(ch);
    else {
      if (!st.length || st.pop() !== pair[ch]) return false;
    }
  }
  return st.length === 0;
}

Template connection

Stack parsing matching.

Reflection

• On an opener, push the closer you expect. A closer that is not the top, or an empty stack, is false.
• ([)] catches a bad order. (() catches a missing close. Is the stack empty at the end?
• The empty string is true. One character is false.