PHP 7 Data Structures and Algorithms

Nested parentheses matching

When we are solving mathematical expressions, the first thing we need to consider is the correctness of nested parentheses. If the parentheses are not nested properly, then calculation might not be possible, or may be wrong. Let us look at some examples:

From the preceding expressions, only the first one is correct; the other two are incorrect, as the parentheses are not nested properly. In order to identify whether or not the parentheses are nested, we can use stack to implement the solution. Here is the pseudo algorithm for the implementation:

valid = true s = empty stack for (each character of the string) {  ...