LeetCode [335] Self Crossing

335. Self Crossing

Hard

You are given an array x of n positive numbers. You start at point (0,0) and moves x[0] metres to the north, then x[1] metres to the west, x[2] metres to the south, x[3] metres to the east and so on. In other words, after each move your direction changes counter-clockwise.

Write a one-pass algorithm with O(1) extra space to determine, if your path crosses itself, or not.

 

Example 1:

┌───┐
│   │
└───┼──>
    │

Input: [2,1,1,2]
Output: true

Example 2:

┌──────┐
│      │
│
│
└────────────>

Input: [1,2,3,4]
Output: false 

Example 3:

┌───┐
│   │
└───┼>

Input: [1,1,1,1] 

Output: true  

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class Solution { public: bool isSelfCrossing(vector<int>& x) { for (int i = 3; i < x.size(); i++) { // last four if (x[i-3] >= x[i-1] && x[i] >= x[i-2]) return true; // last five if (i >= 4 && x[i-1] == x[i-3] && (x[i]+x[i-4] >= x[i-2])) return true; // last six if (i >= 5 && x[i-1] <= x[i-3] && (x[i-1]+x[i-5] >= x[i-3]) && x[i-4] <= x[i-2] && (x[i]+x[i-4] >= x[i-2])) return true; } return false; } };