剑指Offer第七章面试题（Java版）

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面试题49：把字符串转换成整数

将一个字符串转换成一个整数，要求不能使用字符串转换整数的库函数。

public class Solution {    public int StrToInt(String str) {        if (str==null) {            try {                throw new NullPointerException("str is null.");            } catch (NullPointerException e) {                e.printStackTrace();            }        }        if (str.length()==0) {            return 0;        }        char[] chs = str.toCharArray();        int result = 0;        int n=0;        boolean flag = true;        if (chs[n]=='+') {            flag = true;            n++;        }else if (chs[n]=='-') {            flag = false;            n++;        }else if (chs[n]<'0'||chs[n]>'9') {            return 0;        }        while (n<chs.length&&chs[n]=='0') {            n++;        }        for (int i = n; i < chs.length; i++) {            if (chs[i]<'0'||chs[i]>'9') {                return 0;            }            result = result*10+(chs[i]-48);        }        if (!flag) {            result *= -1;        }        return result;    }}

面试题50：树中两个节点的最低公共祖先

import java.util.Stack;public class Demo50 {    //求最低公共祖先    public TreeNode GetLastCommonParent(TreeNode root,            TreeNode treenode1,TreeNode treenode2){        if (root==null||treenode1==null||treenode2==null) {            return null;        }        Stack<TreeNode> stack1 = new Stack<>();        Stack<TreeNode> stack2 = new Stack<>();        boolean treenodeflag1 = false;        boolean treenodeflag2 = false;        treenodeflag1 = GetNodePath(root, treenode1, stack1);        treenodeflag2 = GetNodePath(root, treenode2, stack2);        if (treenodeflag1&&treenodeflag2) {//如果都找到了两条路径            return GetLastCommonNode(stack1,stack2);        }        return null;    }    //求各个节点的路径    public boolean GetNodePath(TreeNode root,TreeNode node,Stack<TreeNode> stack){        //说明找到该路径了        if (root==node) {            return true;        }        //如果是叶节点了        if (root.left==null&&root.right==null) {            return false;        }        //入栈        stack.push(root);        boolean foundleft = false;        boolean foundright = false;        if (root.left!=null) {            foundleft = GetNodePath(root.left,node,stack);            if (foundleft) {                return true;            }        }        if (root.right!=null) {            foundright = GetNodePath(root.right,node,stack);            if (foundright) {                return true;            }        }        if (!foundleft&&!foundright) {            stack.pop();        }        return false;    }    /**     * 查找最后一个公共节点     * @param stack1     * @param stack2     * @return     */    public TreeNode GetLastCommonNode(Stack<TreeNode> stack1,Stack<TreeNode> stack2){        TreeNode lastNode=null;        while (!stack1.isEmpty()&&!stack2.isEmpty()) {            TreeNode node1 = stack1.pop();            TreeNode node2 = stack2.pop();            if (node1==node2) {                lastNode = node1;            }        }        return lastNode;    }}

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