Lowest Common Ancestor III

Analysis:

1) 左子树是不是有LCA(A,B)

2) 右子树是不是有LCA(A,B)

==>

A. 有 返回LCA(A,B)

B. 没有

==>

b1) 如果有遇到A或者B， 返回A或B  

b2)没有遇到返回null

 

注意；要判断AB有没有遇到。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root The root of the binary tree.
     * @param A and B two nodes
     * @return: Return the LCA of the two nodes.
     */
    private class RstType {
        TreeNode node;
        boolean isAExist;
        boolean isBExist;
        public RstType (TreeNode node, boolean isAExist, boolean isBExist) {
            this.node = node;
            this.isAExist = isAExist;
            this.isBExist = isBExist;
        }
    }
    public TreeNode lowestCommonAncestor3(TreeNode root, TreeNode A, TreeNode B) {
        // write your code here
        RstType rst = helper(root, A, B);
        if (rst.isAExist && rst.isBExist) {
            return rst.node;
        } else {
            return null;
        }
    }
    
    RstType helper(TreeNode root, TreeNode A, TreeNode B) {
        if (root == null) {
            return new RstType(null, false, false);
        }
        RstType left = helper(root.left, A, B);
        RstType right = helper(root.right, A, B);
        
        boolean isAExist = left.isAExist || right.isAExist || root == A;
        boolean isBExist = left.isBExist || right.isBExist || root == B;
        
        if (root == A || root == B) {
            return new RstType(root, isAExist, isBExist);
        }
        
        if (left.node != null && right.node != null) { 
            return new RstType(root, isAExist, isBExist);
        } else if (left.node != null) {
            return new RstType(left.node, isAExist, isBExist); //因为右子树为空，所以LCA(左右）就是LCA(左)
        } else if (right.node != null) {
            return new RstType(right.node, isAExist, isBExist);
        } else {
            return new RstType(null, isAExist, isBExist);
        }
        
        //System.out.println(root.val);
        
        
    }
}