Mistake Collections
Common mistakes
While I was solving this problem, I found out myself making some stupid mistakes:
- Wrong return value:
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else {
return {0, -1}; // should be {-1, 0}
}
- Wrong while statement:
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while (i < n*n+1) { // should be n*n
// ...
}
Mistakes do matter. Sometimes I had to waste 30 minutes finding out that a single typo I made or false condition statement was the main problem.
To prevent this:
- Always test a block of code before running the entire program.
- Be aware of iteration statements.
- Don’t change variable names frequently.
Write down all expected outputs
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// Solution 1 (WRONG)
class Solution {
public:
void rotate(vector<int>& nums, int k) {
reverse(nums.begin(), nums.end());
int N = nums.size();
int smallK = k % nums.size();
for (int i = 0; i < smallK; i++) {
swap(nums[i], nums[smallK-i-1]);
}
for (int i = 0; i < N-smallK; i++) {
swap(nums[smallK+i], nums[N-1-i]);
}
}
};
If given nums = [1,2,3,4,5,6,7], k = 3, this code outputs [7,6,5,4,3,2,1]. What is wrong with the code?
Never trust your own code. Test your code by writing expected outputs for every step in for loop:
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// nums = {1,2,3,4,5,6,7}, k = 3
// reversed
nums = {7,6,5,4,3,2,1}
// for (int i = 0; i < smallK; i++) { // smallK = 3
i = 0 : nums = {5,6,7,4,3,2,1}
i = 1 : nums = {5,6,7,4,3,2,1}
i = 2 : nums = {7,6,5,4,3,2,1}
-> Now we know what's wrong.
By writing outputs in each iteration step, we found out that it is swapping elements that have been swapped correctly once again.
The correct solution is:
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// Solution 2 (CORRECT)
class Solution {
public:
void rotate(vector<int>& nums, int k) {
reverse(nums.begin(), nums.end());
int N = nums.size();
int smallK = k % nums.size();
for (int i = 0; i < smallK/2; i++) {
swap(nums[i], nums[smallK-i-1]);
}
for (int i = 0; i < (N-smallK)/2; i++) {
swap(nums[smallK+i], nums[N-1-i]);
}
}
};