20. Valid Parentheses
Problem:
Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
An input string is valid if:
- Open brackets must be closed by the same type of brackets.
- Open brackets must be closed in the correct order.
Note that an empty string is also considered valid.
Example 1:
Input: "()"
Output: true
Example 2:
Input: "()[]{}"
Output: true
Example 3:
Input: "(]"
Output: false
Example 4:
Input: "([)]"
Output: false
Example 5:
Input: "{[]}"
Output: true
Solution:
Stack 101.
Whenever we meet a close bracket, we want to compare it to the last open bracket.
That is why we use stack to store open brackets: first in, last out.
And since there is only bracket characters, the last open bracket happens to be the last character.
/**
* @param {string} s
* @return {boolean}
*/
let isValid = function (s) {
const stack = [];
const pairs = {
"}": "{",
"]": "[",
")": "(",
};
for (const c of s) {
const open = pairs[c];
if (open) {
if (stack.pop() !== open) {
return false;
}
} else {
stack.push(c);
}
}
return stack.length <= 0;
};
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