LeetCode-335.Self Crossing

https://leetcode.com/problems/self-crossing/

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:

Given x = [code][2, 1, 1, 2]

,
┌───┐
│ │
└───┼──>
│

Return true (self crossing)
[/code]

Example 2:

Given x = [code][1, 2, 3, 4]

,
┌──────┐
│ │
│
│
└────────────>

Return false (not self crossing)
[/code]

Example 3:

Given x = [code][1, 1, 1, 1]

,
┌───┐
│ │
└───┼>

Return true (self crossing)
[/code]

相交有三种情况


根据以上三种情况很容易写出代码

bool isSelfCrossing(vector<int>& x)
{
int n = x.size();
for (int i = 3; i < n; i++)
{
if (x[i] >= x[i - 2] && x[i - 1] <= x[i - 3])
return true;
if (i > 3 && x[i - 1] == x[i - 3] && x[i - 2] <= x[i] + x[i - 4])
return true;
if (i > 4 && x[i - 1] <= x[i - 3] && x[i - 4] <= x[i - 2] && x[i - 2] <= x[i] + x[i - 4] && x[i - 3] <= x[i - 1] + x[i - 5])
return true;
}
return false;
}参考 http://www.cnblogs.com/Liok3187/p/5218788.html