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PORed Black Tree in C++, deletion algorithm
 

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  1. PORed Black Tree in C++, deletion algorithm
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    <p>From "Introduction to alghorithms, 2nd edition": Implementation of the deletion algorithm in C++ looks like this: </p> <pre><code>template&lt;class Tree_T, class Node_T&gt; void rb_delete(Tree_T* t,Node_T* z) { Node_T* y = nullptr; Node_T* x = nullptr; if (!get_left(z) || !get_right(z)) { y = z; } else { //y = rb_successor(z); y = rb_predecessor(z); } if (get_left(y)) { x = get_left(y); } else { x = get_right(y); } set_parent(x,get_parent(y)); if (!get_parent(y)) { set_root(t,x); } else { if (y == get_left(get_parent(y))) { set_left(get_parent(y),x); } else { set_right(get_parent(y),x); } } if (y != z) { set_key(z,get_key(y)); set_value(z,get_value(y)); } if(get_color&gt;(y) == Black) { rb_delete_fixup(t,x); } } template&lt;class Tree_T, class Node_T&gt; void rb_delete_fixup(Tree_T* t, Node_T* x) { while (x != get_root(t) &amp;&amp; get_color(x) == Black) { //more code here but it never gets executed! } set_color(x,Black); } </code></pre> <p>The problem is that when I create tree in order 1,2,3,4,5,6,7,8 the tree looks like this:</p> <p><img src="https://i.stack.imgur.com/NVXqd.png" alt="enter image description here"></p> <p>If I were to delete root from this tree I'm getting: </p> <pre><code>void rb_delete(Tree_T* t,Node_T* z)//t is the tree, z is a node with key 4 { Node_T* y = nullptr; Node_T* x = nullptr; if (!get_left(z) || !get_right(z))//z has left &amp;&amp; right so this will get skipped { y = z; } else { //y = rb_successor(z); y = rb_predecessor(z);//to here where y will be set to 3 } if (get_left(y))//this is false { x = get_left(y); } else//so we skipping to this place { x = get_right(y);//and here x goes set to nullptr! } set_parent(x,get_parent(y));//cannot set, x == nullptr if (!get_parent(y))//this is false { set_root(t,x); } else { if (y == get_left(get_parent(y)))//this is false { set_left(get_parent(y),x); } else { set_right(get_parent(y),x);//so here we set right of parent y to nullptr } } if (y != z)//this is true { set_key(z,get_key(y)); set_value(z,get_value(y)); } if(get_color&gt;(y) == Black)//this is true aswell { rb_delete_fixup(t,x);//here we attempting to do fixup but x == nullptr! } } //So we are going into rb_delete_fixup template&lt;class Tree_T, class Node_T&gt; void rb_delete_fixup(Tree_T* t, Node_T* x)//x == nullptr { while (x != get_root(t) &amp;&amp; get_color(x) == Black)//this will get skipped because x == nullptr so no code will be executed within while loop! { //more code here but it never gets executed! } set_color(x,Black);//here x which is nullptr will get colored to Black } </code></pre> <p>This code obviously doesn't work, bear in mind it is implemented line by line from book mentioned at the beginning of this question. </p> <p>Could anyone help me and explain to me how shall I fix it?</p>
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    1. USFloern
    1. USuser336359
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    1. COHave you tried debugging it to see where its behaviour diverges from what you expect?
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      1. USOliver Charlesworth
    2. CO@Oli yes, I've tried and I 'think' that the fact that x is set to nullptr isn't the way it should be, yet I cannot see other possibility really.
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    3. COIn `rb_delete` you're using `y = rb_predecessor(z)` rather than the commented out `y = rb_successor(z)` (I'm 99% sure, having implemented a rb tree from the same book, that this is wrong).
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      1. USuser786653
 

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