[add] LeetCode 404. Sum of Left Leaves

Author: brianway

algorithms-leetcode/README.md

@@ -99,6 +99,7 @@ Leetcode problems classified by company:
 |354|Russian Doll Envelopes|Hard|||
 |376|Wiggle Subsequence|Medium|Greedy/Dynamic Programming|TODO 一题多解|
 |392|Is Subsequence|Easy|Dynamic Programming/Two Pointers|一题多解|
+|404|Sum of Left Leaves|Easy|Binary Tree||
 |416|Partition Equal Subset Sum|Medium|Dynamic Programming,01背包||
 |455|Assign Cookies|Easy|Greedy||
 |474|Ones and Zeroes|Medium|Dynamic Programming,01背包|TODO 二维解法|

algorithms-leetcode/src/main/java/com/brianway/learning/algorithms/leetcode/easy/SumOfLeftLeaves.java

@@ -0,0 +1,44 @@
+package com.brianway.learning.algorithms.leetcode.easy;
+
+import com.brianway.learning.algorithms.leetcode.common.TreeNode;
+
+/**
+ * LeetCode 404. Sum of Left Leaves
+ * Question https://leetcode.com/problems/sum-of-left-leaves/
+ * 关键题设：A leaf is a node with no children.
+ * A left leaf is a leaf that is the left child of another node.
+ *
+ * @auther brian
+ * @since 2022/9/6 00:00
+ */
+public class SumOfLeftLeaves {
+
+    public int sumOfLeftLeaves(TreeNode root) {
+        return 0;
+    }
+
+    /**
+     * 递归
+     * 后序遍历
+     */
+    public class SumOfLeftLeaves0 extends SumOfLeftLeaves {
+        @Override
+        public int sumOfLeftLeaves(TreeNode root) {
+            // 终止条件
+            if (root == null) {
+                return 0;
+            }
+            // 单层逻辑
+            int leftSum = sumOfLeftLeaves(root.left);
+            if (root.left != null && root.left.left == null && root.left.right == null) {
+                // 仅这个if条件下才会有val值贡献进来
+                leftSum = root.left.val;
+            }
+            int rightSum = sumOfLeftLeaves(root.right);
+
+            return leftSum + rightSum;
+        }
+    }
+
+}
+