Stack to the Rescue

The easiest way to accomplish what we want is with a stack data structure, as its push() and pop() methods model the pair-wise opposing symbols in an efficient manner (the push/pop operations for most stack implementations is O(1)).

With a simple stack in place, we can start by iterating through the given string.

At each step, look for any opening/left-side symbol and push it onto the stack. Here's a sample snippet of what that looks like.

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const stack = [];

const str = "[{()}]";

​

for (let i = 0; i < str.length; i++) {

  if (str[i] === "(" || str[i] === "[" || str[i] === "{") {

    stack.push(str[i]);

  }

}

​

console.log(stack);