Right view of binary tree without using queue in Java

问题

HERE is the c++ implementation for right view of binary tree without using queue. When I try to convert it to Java, it is not working. Here is my Java code:

(I think most likely it is because I have not understood the algorithm properly and handling of maxLevel pointers/reference)

public static void rightView(TreeNode tNode){
    int maxLevel = 0;
    rViewUtil(tNode, 1,maxLevel);
}

public static void rViewUtil(TreeNode tNode, int level, int maxLevel){
    if(tNode==null)
        return;
    if(maxLevel < level){
        System.out.print(tNode.value + " ");
        maxLevel = level;
    }
    rViewUtil(tNode.right, level+1, maxLevel);
    rViewUtil(tNode.left, level+1, maxLevel);
}

回答1:

In the spirit of @lifus' answer, but avoiding mutable state, you can use the function return to set maxLevel

public static void rightView(TreeNode tNode){
    int maxLevel = 0;
    rViewUtil(tNode, 1,maxLevel);
}

public static int rViewUtil(TreeNode tNode, int level, int maxLevel){
    if(tNode==null)
        return;
    if(maxLevel < level){
        System.out.print(tNode.value + " ");
        maxLevel = level;
    }
    maxLevel = rViewUtil(tNode.right, level+1, maxLevel);
    maxLevel = rViewUtil(tNode.left, level+1, maxLevel);
    return maxLevel
}

回答2:

Java is pass by value.

i.e. maxLevel = level; doesn't affect other maxLevel values.

In you case, you should remove parameter maxLevel and use static variable instead:

private static int maxLevel;

public static void rightView(TreeNode tNode){
    maxLevel = 0;
    rViewUtil(tNode, 1);
}

public static void rViewUtil(TreeNode tNode, int level){
    ...
}

Note: It's not thread safe

Alternatively, you may use AtomicInteger:

public static void rightView(TreeNode tNode){
    rViewUtil(tNode, 1, new AtomicInteger());
}

public static void rViewUtil(TreeNode tNode, int level, AtomicInteger maxLevel){
    ...
    if(maxLevel.get() < level){
        ...
        maxLevel.set(level);
    }
}

It stores volatile int under the hood.

Note: As @Daniel mentioned, it's in concurrent package and intended for multithreading applications.

Another option is to create your "own" mutable integer or reuse an existing one.

来源：https://stackoverflow.com/questions/28326915/right-view-of-binary-tree-without-using-queue-in-java

标签

java

algorithm

binary-tree