565. Array Nesting

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problem

solution

detect cycle

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class Solution {
public:
    int arrayNesting(vector<int>& nums) {
        int n = nums.size(), ret = 0;
        vector<bool> visited(n, false);
        for(int i=0;i<n;++i){
            int size = 0, j = i;
            while( visited[j] == false){
                visited[j] = true;
                j = nums[j];
                size++;
                
            }
            ret = max(ret, size);
        }
        return ret;
    }
};

option 2 - constant space

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class Solution {
public:
    int arrayNesting(vector<int>& nums) {
        int n = nums.size(), ret = 0;
        for(int i=0;i<n;++i){
            int size = 0, j = i;
            while( nums[j] >= 0){
                int idx = nums[j];
                nums[j] = -1;
                j = idx;
                size++;
                
            }
            ret = max(ret, size);
        }
        return ret;
    }
};

analysis

  • time complexity O(n)
  • space complexity O(1)