This is one of those problems where the clean idea matters more than the amount of code. For 581. Shortest Unsorted Continuous Subarray, the solution in this repository is mainly a two pointers solution.
Guide
What?
The code is easier to read if we treat it as a controlled search through possible states. Instead of trying to be clever immediately, read the code as a sequence of questions:
- What state are we keeping?
- How do we move from one state to the next?
- When do we know the answer is already determined?
For this file, the main tools are: two pointers, stack, greedy.
The notes already sitting in the source point us in the right direction:
- Sorting
- time: O(NlogN), space: O(N)
Guide
When?
Reach for this shape when a direct simulation would work logically but waste time revisiting the same information. The accepted code reduces that pressure by storing exactly the information that remains useful later.
The important function names to track are findUnsortedSubarray.
Guide
Why?
The solution works because it narrows the problem until every update has a clear reason to exist.
- Sorting is used to make local choices comparable, so the later scan does not have to rediscover order.
- The stack stores unfinished context, which is usually the cleanest way to handle nested or monotonic structure.
- The final return is not magic; it is the invariant after the loops or recursion have finished doing their accounting.
Guide
How?
Walk through the solution in this order:
- Start from the smallest reliable state.
- Expand one legal move at a time.
- Cache, count, or merge information as soon as it becomes settled.
- Let the final stored value answer the original question.
The most important competitive-programming habit here is to trust the invariant. Once the invariant is right, the loops become much less scary.
Guide
Complexity
- Time: O(NlogN), space: O(N)
- Space: O(n) in the usual case for auxiliary containers or recursion
Guide
C++ Solution
Your submission
The accepted solution
solution.cpp
01//Sorting
02//Runtime: 80 ms, faster than 58.67% of C++ online submissions for Shortest Unsorted Continuous Subarray.
03//Memory Usage: 27.6 MB, less than 7.63% of C++ online submissions for Shortest Unsorted Continuous Subarray.
04//time: O(NlogN), space: O(N)
05class Solution {
06public:
07 int findUnsortedSubarray(vector<int>& nums) {
08 int n = nums.size();
09 if(n <= 1) return 0;
10
11 vector<int> sorted = nums;
12 sort(sorted.begin(), sorted.end());
13
14 int start = -1, end = -1;
15 for(int i = 0; i < n; ++i){
16 if(nums[i] != sorted[i]){
17 start = i;
18 break;
19 }
20 }
21 if(start == -1) return 0;
22
23 for(int i = n-1; i >= 0; --i){
24 if(nums[i] != sorted[i]){
25 end = i;
26 break;
27 }
28 }
29 return end - start + 1;
30 }
31};
32
33//Approach 2: Better Brute Force
34//TLE
35//211 / 307 test cases passed.
36//time: O(N^2), space: O(1)
37class Solution {
38public:
39 int findUnsortedSubarray(vector<int>& nums) {
40 int n = nums.size();
41 int l = n, r = -1;
42
43 for(int i = 0; i < n; ++i){
44 for(int j = i+1; j < n; ++j){
45 if(nums[j] < nums[i]){
46 l = min(l, i);
47 r = max(r, j);
48 }
49 }
50 }
51
52 //r-l+1 could be negative
53 return max(0, r-l+1);
54 }
55};
56
57//Approach 4: Using Stack
58//Runtime: 72 ms, faster than 63.01% of C++ online submissions for Shortest Unsorted Continuous Subarray.
59//Memory Usage: 28 MB, less than 7.65% of C++ online submissions for Shortest Unsorted Continuous Subarray.
60//time: O(NlogN), space: O(N)
61class Solution {
62public:
63 int findUnsortedSubarray(vector<int>& nums) {
64 int n = nums.size();
65
66 stack<int> stk;
67
68 int l = n, r = -1;
69
70 for(int i = 0; i < n; ++i){
71 //pop all previous index s.t. they are larger than current element
72 //find the min l s.t. nums[l] is not at its right position
73 while(!stk.empty() && nums[stk.top()] > nums[i]){
74 l = min(l, stk.top()); stk.pop();
75 // cout << "l: " << l << ", " << i << endl;
76 // r = i; //this is wrong!
77 }
78
79 stk.push(i);
80 }
81
82 stk = stack<int>(); //!
83 for(int i = n-1; i >= 0; --i){
84 //find the max r s.t. nums[r] is not at its right position
85 while(!stk.empty() && nums[stk.top()] < nums[i]){
86 r = max(r, stk.top()); stk.pop();
87 // cout << "r: " << r << ", " << i << endl;
88 }
89 stk.push(i);
90 }
91
92 // cout << "[" << l << ", " << r << "]" << endl;
93
94 return max(0, r-l+1);
95 }
96};
97
98//Approach 5: Without Using Extra Space
99//Runtime: 52 ms, faster than 94.59% of C++ online submissions for Shortest Unsorted Continuous Subarray.
100//Memory Usage: 26.8 MB, less than 7.65% of C++ online submissions for Shortest Unsorted Continuous Subarray.
101//time: O(N), space: O(1)
102class Solution {
103public:
104 int findUnsortedSubarray(vector<int>& nums) {
105 int n = nums.size();
106 bool flag = false;
107 int minv = INT_MAX, maxv = INT_MIN;
108
109 for(int i = 1; i < n; ++i){
110 //see the first unsorted element
111 if(nums[i] < nums[i-1]) flag = true;
112 //start to record the min element in the unsorted array
113 if(flag) minv = min(minv, nums[i]);
114 }
115
116 flag = false;
117 for(int i = n-2; i >= 0; --i){
118 if(nums[i] > nums[i+1]) flag = true;
119 if(flag) maxv = max(maxv, nums[i]);
120 }
121
122 int l, r;
123 for(l = 0; l < n; ++l){
124 if(nums[l] > minv) break;
125 }
126 //minv should be at l
127
128 for(r = n-1; r >= 0; --r){
129 if(nums[r] < maxv) break;
130 }
131 //maxv should be at r
132
133 return max(r-l+1, 0);
134 }
135};Cost
Complexity
Time
O(NlogN), space: O(N)
Dominated by the main traversal, recursion, or data-structure operations in the code.
Space
O(n) in the usual case for auxiliary containers or recursion
Auxiliary state plus the answer structure where the problem requires one.