Find the Repeated and Missing Values in a Square Grid
Company: Google
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: easy
Interview Round: Other
# Find the Repeated and Missing Values in a Square Grid
You are given a 0-indexed `n * n` integer grid whose values lie in `[1,n^2]`. Exactly one value `a` appears twice, exactly one value `b` is missing, and every other value in that range appears exactly once.
Return the two-element array `[a,b]`: the repeated value first and the missing value second.
### Examples
**Example 1**
```text
Input: grid = [[4,1],[4,3]]
Output: [4,2]
```
The range is `1..4`. Value `4` appears twice, value `2` never appears, and `1` and `3` each appear once.
**Example 2**
```text
Input: grid = [[2,8,1],[6,3,9],[7,1,5]]
Output: [1,4]
```
Among the values `1..9`, only `1` is repeated and only `4` is absent.
### Constraints
- `2 <= n == grid.length == grid[i].length <= 50` for each row `i`.
- `1 <= grid[i][j] <= n * n`.
- Exactly one value in `[1,n^2]` occurs zero times.
- Exactly one value in `[1,n^2]` occurs exactly twice.
- Every other value in `[1,n^2]` occurs at exactly one grid position.
### Clarifying Questions
- Are the rows and columns equal in count? Yes; the grid is square.
- Are there potentially several different missing or repeated values? No; the stated guarantees give exactly one of each.
- Should the answer contain coordinates? No; return the values themselves.
- Can the answer order be reversed? No; repeated comes first, missing second.
- Is an in-place grid modification required? No such requirement is stated; preserve the given method interface and return the two values.
### C++ Interface
```cpp
class Solution {
public:
vector<int> findMissingAndRepeatedValues(vector<vector<int>>& grid) {
}
};
```
### Reasoning Checkpoints
- Use the full value range from one through n squared, including both endpoints.
- Distinguish value frequencies from grid coordinates.
- Apply the exact input guarantees and preserve the required answer order.
```hint Count occurrences across the grid
After counting every cell, which frequency identifies the repeated value, and which identifies the missing one?
```
Overview: Find the single repeated value and single missing value in a square grid containing the numbers from one through n squared, returning repeated before missing.