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Q437PathSumIII.java
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Q437PathSumIII.java
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import util.TreeNode;
import java.util.HashMap;
/**
* @author ahscuml
* @date 2018/11/14
* @time 20:35
*/
public class Q437PathSumIII {
/**
* 测试函数
*/
public static void main(String[] args) {
TreeNode treeNode1 = new TreeNode(1);
TreeNode treeNode2 = new TreeNode(2);
TreeNode treeNode3 = new TreeNode(3);
TreeNode treeNode4 = new TreeNode(4);
TreeNode treeNode5 = new TreeNode(5);
treeNode1.left = treeNode2;
treeNode1.right = treeNode3;
treeNode2.left = treeNode4;
treeNode2.right = treeNode5;
System.out.println(pathSum(treeNode1, 7));
}
public static int pathSum(TreeNode root, int sum) {
// 从根节点到这个节点和为K
HashMap<Integer, Integer> preSum = new HashMap();
preSum.put(0, 1);
return helper(root, 0, sum, preSum);
}
public static int helper(TreeNode root, int currSum, int target, HashMap<Integer, Integer> preSum) {
if (root == null) {
return 0;
}
currSum += root.val;
int res = preSum.getOrDefault(currSum - target, 0);
preSum.put(currSum, preSum.getOrDefault(currSum, 0) + 1);
res += helper(root.left, currSum, target, preSum) + helper(root.right, currSum, target, preSum);
// 递归完成后代表回溯,所以还要减去一个
preSum.put(currSum, preSum.get(currSum) - 1);
return res;
}
}