# Backspace String Compare

`Two Pointers`, `String`

**Easy**

Given two strings `S` and`T`, return if they are equal when both are typed into empty text editors.`#`means a backspace character.

**Example 1:**

```
Input: S = "ab#c", T = "ad#c"
Output: true

Explanation: Both S and T become "ac".
```

**Example 2:**

```
Input: S = "ab##", T = "c#d#"
Output: true

Explanation: Both S and T become "".
```

**Example 3:**

```
Input: S = "a##c", T = "#a#c"
Output: true

Explanation: Both S and T become "c".
```

**Example 4:**

```
Input: S = "a#c", T = "b"
Output: false

Explanation: S becomes "c" while T becomes "b".
```

**Note**:

1. `1 <= S.length <= 200`
2. `1 <= T.length <= 200`
3. `S` and `T` only contain lowercase letters and `'#'` characters.

**Follow up:**

* Can you solve it in `O(N)` time and `O(1)` space?

## Solution & Analysis

模拟字符串的操作，生成最终字符串并比较

```java
class Solution {
    public boolean backspaceCompare(String S, String T) {
        return build(S).equals(build(T));
    }

    public String build(String S) {
        Stack<Character> ans = new Stack();
        for (char c: S.toCharArray()) {
            if (c != '#')
                ans.push(c);
            else if (!ans.empty())
                ans.pop();
        }
        return String.valueOf(ans);
    }
}
```

但是难度在于用O(1)的空间复杂度。这里就可以想到用Two Pointer。

```java
class Solution {
    public boolean backspaceCompare(String S, String T) {
        int i = S.length() - 1, j = T.length() - 1;
        int skipS = 0, skipT = 0;

        while (i >= 0 || j >= 0) { // While there may be chars in build(S) or build (T)
            while (i >= 0) { // Find position of next possible char in build(S)
                if (S.charAt(i) == '#') {skipS++; i--;}
                else if (skipS > 0) {skipS--; i--;}
                else break;
            }
            while (j >= 0) { // Find position of next possible char in build(T)
                if (T.charAt(j) == '#') {skipT++; j--;}
                else if (skipT > 0) {skipT--; j--;}
                else break;
            }
            // If two actual characters are different
            if (i >= 0 && j >= 0 && S.charAt(i) != T.charAt(j))
                return false;
            // If expecting to compare char vs nothing
            if ((i >= 0) != (j >= 0))
                return false;
            i--; j--;
        }
        return true;
    }
}
```

Rewrite

```java
class Solution {
    public boolean backspaceCompare(String S, String T) {
        int i = S.length() - 1;
        int j = T.length() - 1;
        int skipS = 0; 
        int skipT = 0;

        while (i >= 0 || j >= 0) {
            while (i >= 0) {
                if (S.charAt(i) == '#') {
                    skipS++;
                    i--;
                } else if (skipS > 0) {
                    skipS--;
                    i--;
                } else {
                    break;
                }
            }

            while (j >= 0) {
                if (T.charAt(j) == '#') {
                    skipT++;
                    j--;
                } else if (skipT > 0) {
                    skipT--;
                    j--;
                } else {
                    break;
                }
            }

            if (i >= 0 && j >= 0 && S.charAt(i) != T.charAt(j)) {
                return false;
            }

            if (i >= 0 && j < 0 || i < 0 && j >= 0) {
                return false;
            }

            i--;
            j--;
        }
        return true;
    }
}
```

## Reference

<https://leetcode.com/problems/backspace-string-compare/solution/>


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