1003. Check If Word Is Valid After Substitutions

Problem

Given a string s, determine if it is valid.

A string s is valid if, starting with an empty string t = "", you can **transform *t* into **s after performing the following operation *any number of times*:

• Insert string "abc" into any position in t. More formally, t becomes tleft + "abc" + tright, where t == tleft + tright. Note that tleft and tright may be empty.

Return true **if *s* is a valid string, otherwise, return** false.

  Example 1:

Input: s = "aabcbc"
Output: true
Explanation:
"" -> "abc" -> "aabcbc"
Thus, "aabcbc" is valid.

Example 2:

Input: s = "abcabcababcc"
Output: true
Explanation:
"" -> "abc" -> "abcabc" -> "abcabcabc" -> "abcabcababcc"
Thus, "abcabcababcc" is valid.

Example 3:

Input: s = "abccba"
Output: false
Explanation: It is impossible to get "abccba" using the operation.

  Constraints:

• 1 <= s.length <= 2 * 10^4

• s consists of letters 'a', 'b', and 'c'

Solution (Java)

class Solution {
    public boolean isValid(String s) {
        Deque<Character> stack = new ArrayDeque<>();
        for (char c : s.toCharArray()) {
            if (c == 'c') {
                if (stack.isEmpty() || stack.pop() != 'b') {
                    return false;
                }
                if (stack.isEmpty() || stack.pop() != 'a') {
                    return false;
                }
            } else {
                stack.push(c);
            }
        }
        return stack.isEmpty();
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).