# Identify and mark unmatched parenthesis in an expression

Given an expression, find and mark matched and unmatched parenthesis in it. We need to replace all balanced opening parenthesis with 0, balanced closing parenthesis with 1 and all unbalanced with -1.

Examples:

```Input : ((a)
Output : -10a1

Input : (a))
Output : 10a-1

Input  : (((abc))((d)))))
Output : 000abc1100d111-1-1```

The idea is based on stack. We run a loop from the start of the string upto end and for every ‘(‘, we push it into stack. If stack is empty and we encounter an closing bracket ‘)’ the we replace -1 at that index of the strig. Else we replace all opening brackets ‘(‘ with 0 and closing brackets with 1. Then pop from the stack.

`// CPP program to mark balanced and unbalanced`
`// parenthesis.`
`#include <bits/stdc++.h>`
`using` `namespace` `std;`
`void` `identifyParenthesis(string a)`
`{`
`    ``stack<``int``> st;`
`    ``// run the loop upto end of the string`
`    ``for` `(``int` `i = 0; i < a.length(); i++) {`
`        ``// if a[i] is opening bracket then push `
`        ``// into stack`
`        ``if` `(a[i] == ``'('``) `
`            ``st.push(i);`
`        `
`        ``// if a[i] is closing bracket ')' `
`        ``else` `if` `(a[i] == ``')'``) {`
`            ``// If this closing bracket is unmatched`
`            ``if` `(st.empty()) `
`                ``a.replace(i, 1, ``"-1"``);`
`            `
`            ``else` `{`
`                ``// replace all opening brackets with 0`
`                ``// and closing brackets with 1`
`                ``a.replace(i, 1, ``"1"``);`
`                ``a.replace(st.top(), 1, ``"0"``);`
`                ``st.pop();`
`            ``}`
`        ``}`
`    ``}`
`    ``// if stack is not empty then pop out all`
`    ``// elements from it and replace -1 at that`
`    ``// index of the string`
`    ``while` `(!st.empty()) {`
`        ``a.replace(st.top(), 1, ``"-1"``);`
`        ``st.pop();`
`    ``}`
`    ``// print final string`
`    ``cout << a << endl;`
`}`
`// Driver code`
`int` `main()`
`{`
`    ``string str = ``"(a))"``;`
`    ``identifyParenthesis(str);`
`    ``return` `0;`
`}`

Output:

```0a1-1

```

Disclaimer: This does not belong to TechCodeBit, its an article taken from the below
source and credits.
source and credits:http://www.geeksforgeeks.org/identify-mark-unmatched-parenthesis-expression/
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