Check If Word Is Valid After Substitutions LeetCode solution in Java

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class Solution {
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  public boolean isValid(String s) {
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    // Lets see how we can solve that as we know we have only abc in string.
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    // Like           aabcbc
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    // See as that   ((b)b) Think a is '(' and c is ')'.
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    // If a string is made by using abc only we can remove abc to make it empty also.
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    // Think in Reverse Way.
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    Stack<Character> stack = new Stack<>();
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    char[] arr = s.toCharArray();
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    for (int i = 0; i < arr.length; i++) {
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      // We have to work only when we get ')' means c.
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      if (arr[i] == 'c') {
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        // If we at c means we have 2 elements before us a and b.
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        // When we first pop we get b at second pop we get a
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        // If this all hold true we will delete a and b we are not adding c so c also
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        if (stack.size() >= 2 && stack.pop() == 'b' && stack.pop() == 'a') {
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        } else {
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          // If anywhere we get false in any condition that means this is not a
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          //     valid set i.e. abc pattern is not present.
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          return false;
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        }
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      } else {
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        // For a and b we simply add.
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        stack.push(arr[i]);
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      }
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    }
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    // If we have only abc pattern the stack will become empty.
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    return stack.size() == 0;
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  }
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}