What is the time and space complexity of Backspace String Compare?

Time complexity: O(n + m). Space complexity: O(1).

Backspace String Compare

Easy

Concept

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Solution Guide: Backspace String Compare

Approach

Process both strings from the end. For each string, skip characters when encountering backspaces by counting consecutive # symbols. Compare the next valid characters from both strings. If they differ at any point, the strings are not equal.

Complexity Analysis

Time: O(n + m)
Space: O(1)

Pattern

Two Pointers (Reverse Traversal)

Why It Works

Processing from the end allows us to handle backspaces without extra space. Each # cancels the preceding character, and counting skips handles consecutive backspaces correctly.

Updated Feb 2026