41.first-missing-positive

Statement

``````输入：nums = [1,2,0]

``````

``````输入：nums = [3,4,-1,1]

``````

``````输入：nums = [7,8,9,11,12]

``````

• `1 <= nums.length <= 5 * 105`
• `-231 <= nums[i] <= 231 - 1`

Given an unsorted integer array `nums`, return the smallest missing positive integer.

You must implement an algorithm that runs in `O(n)` time and uses constant extra space.

Example 1:

``````Input: nums = [1,2,0]
Output: 3
``````

Example 2:

``````Input: nums = [3,4,-1,1]
Output: 2
``````

Example 3:

``````Input: nums = [7,8,9,11,12]
Output: 1
``````

Constraints:

• `1 <= nums.length <= 5 * 105`
• `-231 <= nums[i] <= 231 - 1`

Solution

``````#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define endl "\n"
#define fi first
#define se second
#define all(x) begin(x), end(x)
#define rall rbegin(a), rend(a)
#define bitcnt(x) (__builtin_popcountll(x))
#define complete_unique(a) a.erase(unique(begin(a), end(a)), end(a))
#define mst(x, a) memset(x, a, sizeof(x))
#define MP make_pair

using ll = long long;
using ull = unsigned long long;
using db = double;
using ld = long double;
using VLL = std::vector<ll>;
using VI = std::vector<int>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;

using namespace __gnu_pbds;
using namespace std;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
const ll mod = 1e9 + 7;

template <typename T, typename S>
inline bool chmax(T &a, const S &b) {
return a < b ? a = b, 1 : 0;
}

template <typename T, typename S>
inline bool chmin(T &a, const S &b) {
return a > b ? a = b, 1 : 0;
}

#ifdef LOCAL
#include <debug.hpp>
#else
#define dbg(...)
#endif

class Solution {
public:
int firstMissingPositive(vector<int> &nums) {
int n = nums.size();
nums.push_back(INT_MAX);

for (auto &c : nums) {
if (c <= 0) {
c = INT_MAX;
}
}

for (auto &c : nums) {
if (abs(c) <= n && nums[abs(c)] > 0) {
nums[abs(c)] *= -1;
}
}

for (int i = 1; i <= n; i++) {
if (nums[i] > 0) {
return i;
}
}

return n + 1;
}
};

#ifdef LOCAL

int main() {
return 0;
}

#endif
``````